Single-crystalline nanoporous Nb2O5 nanotubes
© Liu et al; licensee Springer. 2011
Received: 8 October 2010
Accepted: 14 February 2011
Published: 14 February 2011
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  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.
Nanomaterials, which have received a wide recognition for their size- and 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 . 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 surface-to-volume ratio. However, it still remains a big long-term 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 . 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 (E g = 3.4 eV), Nb2O5 is the most thermodynamically stable phase among various niobium oxides . 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 , 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  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.
Nb2O5 nanorod precursors
Nb2O5 nanorods were prepared via hydrothermal technique in a Teflon-lined stainless steel autoclave. In a typical synthesis of 1 D Nb2O5 nanorods, freshly prepared niobic acid (the detailed synthesis processes of niobic acid from Nb2O5 has been described in previous studies by the authors [22–25]) was added to the mixture of ethanol/deionized water. Subsequently, the white suspension was filled into a Teflon-lined stainless steel autoclave. The autoclave was maintained at 120-200°C for 12-24 h without shaking or stirring during the heating period and then naturally cooled down to room temperature. A white precipitate was collected and then washed with deionized water and ethanol. The nanorod precursors were dried at 60°C in air.
Single-crystalline nanoporous Nb2O5 nanotubes
In a typical transformation, 0.06-0.20 g of the obtained Nb2O5 nanorods was added to 20-40 ml deionized water at room temperature. 2-8 mmol NH4F was then added while stirring. Afterward, the mixture was transferred into a Teflon-lined stainless steel autoclave and kept inside an electric oven at 120-180°C for 12-24 h. Finally, the resulting Nb2O5 nanotubes were collected, and washed with deionized water and ethanol, and finally dried at 60°C in air.
The collected products were characterized by an X-ray diffraction (XRD) on a Rigaku-DMax 2400 diffractometer equipped with the graphite monochromatized Cu Kα radiation flux at a scanning rate of 0.02°s-1. Scanning electron microscopy (SEM) analysis was carried using a JEOL-5600LV scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDS) microanalysis of the samples was performed during SEM measurements. The structures of these nanorod precursors and nanotube products were investigated by means of transmission electron microscopy (TEM, Philips, TecnaiG2 20). UV-Vis adsorption spectra were recorded on UV-Vis-NIR spectrophotometer (JASCO, V-570). The photoluminescence (PL) spectrum was measured at room temperature using a Xe lamp with a wavelength of 325 nm as the excitation source.
Results and discussion
where α, v, E g, and A are the absorption coefficient, light frequency, band gap energy, and a constant, respectively [16, 26]. The band gap energy (E g) of Nb2O5 can be defined by extrapolating the rising part of the plots to the photon energy axis. The estimated band gaps of Nb2O5 nanotubes and nanorods are 3.97 and 3.72 eV, respectively (Figure 9b), which are both larger than the reported value (3.40 eV) of bulk crystals . The blue shift (approximately 0.25 eV) of the absorption edge for the porous nanotubes compared to solid nanorods exhibits a possible quantum size effect in the orthorhombic nanoporous Nb2O5 nanotubes . Wavelength and intensity of absorption spectra of Nb2O5 nanocrystals depend on the size, crystalline type and morphology of the Nb2O5 nanocrystals. If their size is smaller, then the absorption spectrum of Nb2O5 nanocrystals becomes blue shifted. The spectral changes are observed because of the formation of nanoporous thin-walled tubular nanomaterials, similar to the previous research result .
In summary, we have elucidated a new preferential-etching synthesis for single-crystalline nanoporous Nb2O5 nanotubes. The shell of resulting nanotubes possesses dense nanopores with size of several nanometers. The formation mechanism of single-crystalline nanoporous nanotubes is mainly due to the preferential etching along c-axis and slow etching along the radial directions. The as-obtained Nb2O5 nanorod precursors and nanotube products can be used as templates for synthesis of 1 D niobate nanostructures. These single-crystalline nanoporous Nb2O5 nanotubes might find applications in catalysis, nanoscale electronics, optoelectronics, and biochemical-sensing devices.
Energy-dispersive X-ray spectroscopy
Scanning electron microscopy.
The financial support of the National Natural Science Foundation of China (Grant Nos. 50872016, 20973033) is acknowledged.
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