Synthesis of Nb 2 O 5 Nanorods by a Soft Chemical Process

Single crystalline Nb2O5 nanorods have been successfully synthesized by a soft chemical process, in which only metal Nb powder and water were used as the starting materials. The synthesized Nb2O5 nanorods are highly crystalline and their growth is along [001] direction. The diameter of the nanorods is found to be ca. 50 nm and their lengths up to several micrometers. Based on the experimental results of XRD, SEM, and TEM measurements, the possible mechanism for the formation of Nb2O5 nanorods was discussed.


Introduction
Since the discovery of carbon nanotubes in 1991 [1], onedimensional (1D) nanomaterials including nanorods, nanotubes, nanowires, nanofibers, nanobelts, and nanoribbons have attracted much attention due to their physical and chemical properties different from those of bulk materials [2].In the past decades, a large number of 1D oxide nanomaterial have been synthesized, such as TiO 2 [3], MnO 2 [4], ZnO [5], SnO 2 [6], VO x [7], MoO 3 [8], Ga 2 O 3 [9], Ta 2 O 5 [10], In 2 O 3 [11], and Nb 2 O 5 [12].Among them, Nb 2 O 5 is an important semiconductor oxide with a wide band gap [13] and had been widely used in electrochromic devices [14], catalysts [15], chemical sensors [16], optical filters [17], solar cells [18], and lithium batteries [19].As is well known, Nb 2 O 5 has many polymorphic forms based on the octahedrally coordinated niobium atoms [20].These polymorphs are identified with a variety of prefixes [21] such as B-Nb 2 O 5 (PdF 3 rutile structure), H-Nb 2 O 5 (ReO 3type block, 3 × 3 or 3 × 5 octahedra), and N-Nb 2 O 5 (ReO 3 -type block, 4 × 4 octahedral).Among the niobium oxides, Nb 2 O 5 is the most stable and exhibits the excellent chemical stability and corrosion resistance in both acid and base media [22].So far, niobium oxide fibers have been prepared by using an electrospinning method [23].Mozetič et al. [24] has synthesized Nb 2 O 5 nanowires via a cold plasma treatment in the presence of a high neutral oxygen flux.Niobia-phase nanorods [25] were obtained by the hydrothermal treatment of a niobium peroxo complex precursor at 140 • C. Hu et al. [26] reported the synthesis of Nb 2 O 5 nanocables using NbCl 5 as a precursor.Kobayashi et al. [27] prepared Nb 2 O 5 nanotubes using the layered K 4 Nb 6 O 17 as a precursor, in which K 4 Nb 6 O 17 was formed at over 1050 • C. Nb 2 O 5 nanotubes have also been obtained using HF solution as a reactant [28].Using amorphous Nb 2 O 5 • nH 2 O as a precursor, Nb 2 O 5 nanorods [29] were formed at a high temperature.More recently, Nb 2 O 5 nanobelts were synthesized by using a hydrothermal route [30].To the best of our knowledge, however, the synthesis of niobium oxide with 1D nanostructure has not been reported by using a soft chemical process.Here, we first fabricated Nb 2 O 5 nanorods starting from metal Nb powder by a soft chemical process, without templates or catalysts, and using only the raw material and water.Furthermore, the possible mechanism for the formation of nanorods was discussed according to the experimental results.

Experimental
A soft chemical process was developed to prepare Nb 2 O 5 nanorods.In a typical procedure, 0.1 g of the commercial metal Nb powder was dispersed into 40 mL distilled water and stirred, then transferred into a 50 mL Teflon-lined autoclave, and kept in an oven at 200 • C for 3 to 30 days.The final products were washed with distilled water and then dried in the air.Scanning electron (SEM) and transmission electron microscopy (TEM) were taken on a Philip-XL30 instrument and a JEOL 2010 instrument, respectively.X-ray diffraction (XRD) pattern was recorded on a PANalytical X'Pert spectrometer using the Co Kα radiation (λ = 1.78897Å) and the data would be changed to Cu Kα data.

Results and Discussion
Figure 1 shows the SEM images of the raw Nb powder and the products obtained at 200 • C for different reaction times.It clearly shows that the SEM morphology of the products is entirely different from the raw material Nb metal powder.Only particles with different sizes were detected in the raw material, as shown in Figure 1(a).After the raw Nb metal powder was treated in H 2 O at 200 • C for 3 days, a large number of sheet-like and nanorod-like products were observed, as depicted in Figure 1(b).With increasing reaction time, numerous nanorods with a bundle-like structure were formed and the sheet-like products disappeared, as depicted in Figures 1(c)-1(f).It can also be found that the length of these nanorods increased significantly with reaction time.These nanorods lie close to each other and their lengths up to several micrometers.
The morphologies of the product can be further confirmed by TEM measurements.Figure 2 to a d-pacing of 0.393 and 0.315 nm, respectively.The typical selected area electron diffraction (SAED) taken from a single nanorod is shown in Figure 2b (inset).The pattern exhibits broadened and strong spots along the growth direction of the nanorods which can be attributed to the essentially asymmetric 1D nature of the long and thin nanorods.These results also indicate that the growth of nanorods is along [001] direction.XRD measurement was used to identify the crystalline structure of the product, as shown in Figure 3.Besides raw Nb metal, the main phase detected was Nb 2 O 5 which could be indexed to Nb 2 O 5 (JPCDS 27-1313) with an orthorhombic structure.This result is in agreement with the TEM observations.Based on the experimental results of XRD, SEM, and TEM measurements, a possible model for the formation of Nb 2 O 5 nanorods is suggested as follows: (i) the surface of the metal Nb powder was oxidized by oxygen in water under the hydrothermal conditions, and the sheetlike products were formed; (ii) the sheet-like products were splitted in order to release strong stress and lower the total energy, and then the nanorods were formed.Therefore, the formation of bundle-like structure can be contributed to the fact that the splitting of the sheets-like products is complete.
In the present work, metal Nb powder was used as a starting material and reacted with water under the hydrothermal conditions to yield single crystalline Nb 2 O 5 nanorods.Although the purity of Nb 2 O 5 nanorods in the final product is still to be improved, the synthetic route is very simple, in which templates or catalysts were not introduced into the reaction system.Comparing with the normal hydrothermal process, such a synthetic route took a long reaction time.This might be related to that the hydrothermal reaction was accelerated in the presence of templates or catalysts.

Conclusions
In summary, single crystalline Nb 2 O 5 nanorods were successfully synthesized by a soft chemical process, in which the metal Nb powder and water were used as the precursors.The synthesized nanorods are highly crystalline and their growth direction is along [001].The diameter of the nanorods is found to be ca.50 nm and their lengths up to several micrometers.Furthermore, the work is underway to optimize the process and increase purity of nanorods in the product.Compared with other synthetic routes, no any catalysts or templates were introduced into the reaction system.We believe that the synthetic route of Nb 2 O 5 nanorods from the metal Nb powder has the potential to prepare 1D nanostructural metal oxides.