Single-crystalline nanoporous Nb2O5 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 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][2][3][4][5][6][7][8]. Among them, one-dimensional (1D) tubular nanostructures with hollow interiors have attracted tremendous research interest since the discovery of carbon nanotubes [1,[9][10][11][12][13][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 Nb 2 O 5 nanotubes with NH 4 F as an etching reagent, which can be easily transformed from Nb 2 O 5 nanorod precursors is presented.
As a typical n-type wide bandgap semiconductor (E g = 3.4 eV), Nb 2 O 5 is the most thermodynamically stable phase among various niobium oxides [16]. Nb 2 O 5 has attracted great research interest due to its remarkable applications in gas sensors, catalysis, optical devices, and Li-ion batteries [9][10][11][16][17][18][19][20][21]. Even monoclinic Nb 2 O 5 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 Nb 2 O 5 nanotubes, according to the preferential growth habit along [001] of Nb 2 O 5 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
Materials synthesis Nb 2 O 5 nanorod precursors Nb 2 O 5 nanorods were prepared via hydrothermal technique in a Teflon-lined stainless steel autoclave. In a typical synthesis of 1 D Nb 2 O 5 nanorods, freshly prepared niobic acid (the detailed synthesis processes of niobic acid from Nb 2 O 5 has been described in previous studies by the authors [22][23][24][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 Nb 2 O 5 nanotubes
In a typical transformation, 0.06-0.20 g of the obtained Nb 2 O 5 nanorods was added to 20-40 ml deionized water at room temperature. 2-8 mmol NH 4 F 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 Nb 2 O 5 nanotubes were collected, and washed with deionized water and ethanol, and finally dried at 60°C in air.

Materials characterization
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
Typical XRD pattern of the Nb 2 O 5 nanorod precursors obtained from the ethanol-water system shown in Figure 1 exhibits diffraction peaks corresponding to the orthorhombic Nb 2 O 5 with lattice constants of a = 3.607 Å and c = 3.925 Å (JCPDS no. 30-0873). No diffraction peaks arising from impurities such as NbO 2 were detected, indicating the high purity of these precursor nanorods. The morphology of these precursor products was observed by means of SEM and TEM.   nanorods with the diameter 300-600 nm and the length 2-4 μm. The bottom inset of Figure 2b shows typical TEM image of a single solid Nb 2 O 5 nanorod, demonstrating that the nanorod have a diameter of~300 nm and length of approximately 2 μm, which is in agreement with the SEM observations. The HRTEM image (the top inset of Figure 2b) taken from the square area exhibits clear lattice fringes, indicating that the nanorod is highly crystallized. The spacing of 0.39 nm corresponds to the (001) planes of Nb 2 O 5 , which shows that these precursor nanorods grow along the [001] direction.
After the hydrothermal process along with an interface reaction, Nb 2 O 5 nanotubes were obtained with F --assisted etching treatment. The XRD pattern shown in Figure 3a reveals a pure phase, and all the diffraction peaks are very consist with that of nanorod precursors and the reported XRD profile of the orthorhombic Nb 2 O 5 (JCPDS no. 30-0873). EDS analysis was used to determine the chemical composition of an individual nanotube. The result shows that these nanotube products contain only Nb and O elements, and their atomic ratio is about 2:5, which is in agreement with the stoichiometric ratio of Nb 2 O 5 . The EDS results clearly confirm that F was not doped into these nanotubes (Figure 3b).
The morphology and structure of the finally nanoporous nanotubes were first evaluated by SEM observation. The representative SEM image in Figure 4a  implying the finally formed nanotubes well resemble the shape and size of Nb 2 O 5 nanorod precursors. The detailed structure information is supported by the highmagnification image shown in Figure 4b, which shows some typical nanotubes with thin walls. For accurately revealing the microstructure of these nanotubes, TEM observation was performed on these nanotubes. Figure  5a shows a typical TEM image of these special nanostructured Nb 2 O 5 . These nanotubes have a hollow cavity and two closed tips. A magnified TEM image of some Nb 2 O 5 nanotubes is presented in Figure 5b. It can been see that the nanotube surface is highly nanoporous and coarse, composed of dense nanopores. SAED pattern obtained from them by TEM shows they are single-crystalline, as seen in the typical pattern in Figure 5b (inset). The nanoporous characterization of these single-crystalline nanotubes was further verified by a higher-magnified TEM image (Figure 5c). The single-crystalline nature of the nanotubes is further indicated by the Nb 2 O 5 lattice which can be clearly seen in the HRTEM image of the surface of a nanoporous nanotube. Though it is difficult to directly observe by TEM, since the observed image is a two-dimensional projection of the nanotubes, Figure 5d shows dense nanopores around which the Nb 2 O 5 lattice is continuous. The diameter of the nanopores appears to be 2-4 nm, and the growth direction of these nanoporous nanotubes is [001], just the same as nanorod precursors. During the hydrothermal process of Nb 2 O 5 nanorod precursors, the formation of single-crystalline nanoporous nanotubes can be ascribed to preferential-etching of single-crystalline nanorods. In hydrothermal aqueous NH 4 F solution, HF were formed by the hydrolysis of NH 4 + and were further reacted with Nb 2 O 5 to form soluble niobic acid. The etching of nanorods in this study preferentially begins at the central site of the nanorod, which might be because the central site has high activity or defects both for growth and for etching. Further etching at the center of nanorod leads to its splitting, and the atom in the (001) planes are removed at the next process, causing the formation of the tubular structure. Furthermore, during the etching process, these newly generated soluble niobic acid diffused into the reaction solution from the central of the precursor nanorods, leaving dense nanopores on the shell of nanotubes with closed tips. For verifying such preferential-etching formation mechanism, HF solution as an etching reagent was directly adopted. Figure 6 shows the morphology and structure of Nb 2 O 5 products, which exhibit that hollow tuber-like nanostructures can also be achieved. However, the asobtained Nb 2 O 5 products are broken or collapsed nanotubes, which is ascribed to the fast etching rate of HF reagent. The diameter of nanoporous nanotubes can be tunable by adjusting the diameter of precursor nanorods. We can thus obtain different diameters of Nb 2 O 5 nanotubes, which could meet various demands of nanotubes toward practical applications. For example, when Nb 2 O 5 nanorods with a smaller diameter (approximately 200 nm) were adopted as precursors, the corresponding Nb 2 O 5 nanotubes with similar sized nanotubes were achieved (Figure 7). These Nb 2 O 5 nanotubes and nanorods can be used as versatile templates to fabricate MNbO 3 (M = Li, Na, K) nanotubes and nanorods. For example, when Nb 2 O 5 nanorod precursors directly reacted with LiOH at high temperature, LiNbO 3 nanorods were immediately achieved. As shown in Figure 8a, b, the morphology of Nb 2 O 5 templates is preserved. XRD pattern of the calcination products (Figure 8c) clearly shows the pure-phase LiNbO 3 ferroelectric materials. These LiNbO 3 nanorods were obtained through calcination of Nb 2 O 5 and LiOH with appropriate amount ratios at  500°C for 4 h. This calcination method is general and versatile, and it can be applied to fabricate other niobate materials such as NaNbO 3 and KNbO 3 . The optical properties of these Nb-based nanomaterials (LiNbO 3 , NaNbO 3 , and KNbO 3 ) are shown in Figure  S1 in Additional file 1). UV-Vis adsorption measurement was used to reveal the energy structure and optical property of the as-prepared Nb 2 O 5 nanorods and finally porous nanotube products. UV-Vis adsorption spectra of Nb 2 O 5 nanorods and nanotubes are presented in Figure 9a. It can be seen from Figure 9a that the structure transformation from solid nanorods to nanoporous nanotubes is accompanied by distinct changes in the UV-Vis spectra because of the significant difference in shape between nanorod precursors and nanotube products. As a direct band gap semiconductor, the optical absorption near the band edge follows the formula 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 Nb 2 O 5 can be defined by extrapolating the rising part of the plots to the photon energy axis. The estimated band gaps of Nb 2 O 5 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 [10]. 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 Nb 2 O 5 nanotubes [10]. Wavelength and intensity of absorption spectra of Nb 2 O 5 nanocrystals depend on the size, crystalline type and morphology of the Nb 2 O 5 nanocrystals. If their size is smaller, then the absorption spectrum of Nb 2 O 5 nanocrystals becomes blue shifted. The spectral changes are observed because of the formation of nanoporous thinwalled tubular nanomaterials, similar to the previous research result [10].

Conclusions
In summary, we have elucidated a new preferential-etching synthesis for single-crystalline nanoporous Nb 2 O 5 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 Nb 2 O 5 nanorod precursors and nanotube products can be used as templates for synthesis of 1 D niobate nanostructures. These single-crystalline nanoporous Nb 2 O 5 nanotubes might find applications in catalysis, nanoscale electronics, optoelectronics, and biochemical-sensing devices.

Additional material
Additional file 1: Figure S1 UV-Vis (a) and PL (b) spectra of Nbbased nanomaterials. PL spectra were obtained with an excitation wavelength of 325 nm measured at room temperature.