Synthesis of single-crystalline niobate nanorods via ion-exchange based on molten-salt reaction.

One-dimensional (1D) nanostructures of ternary complex oxides have drawn intensive interest because of their novel size-dependent properties, such as ferroelectric,1a multiferroic,1b and superconductivity.1c Various synthetic approaches, such as aqueous route by employing hydrothermal reaction2 or templates,3 molten-salt synthesis,4 and composite-hydroxide-mediated synthesis5 have been developed for the synthesis of different kinds of ternary metal oxides 1D nanostructures, for example, alkali titanates,6 peroskite-type ferroelectric titanates,7,4c vanadates,8 niobates,9 and ABO4-type chromates and tungstates.10 Alkali and alkali earth niobates might exhibit excellent nonlinear optical and dielectric properties, on the basis of which novel nanodevices might be designed by utilizing their 1D nanostructures. For example, subwavelength optical microscopy by employing tunable nanometric light source based on KNbO3 nanowires were developed by Yang and co-workers.11 Despite the attractive applications of niobates, there are only a few reports on the synthesis of niobate 1D nanostructures by employing the hydrothermal approach.9 In this Communication, we have developed a facile ionexchange approach for the synthesis of single-crystal sodium and calcium niobates nanorods based on molten-salt reaction between K2Nb8O21 nanowires and molten sodium or calcium salts. The proposed synthetic route is suggested to be a self-sacrificing templated process. The starting template of K2Nb8O21 nanowires was prepared by calcinations of Nb2O5 in molten potassium chloride, similar to our previous report on the synthesis of K2Nb8O21 nanoribbons.12 Although previously ion-exchange has already been explored for the synthesis of titanate nanowires,13 these works were mainly based on an aqueous solution reaction, which is timeconsuming and thus not suitable for large-scale synthesis. The proposed ion-exchange approach based on molten-salt synthesis is thus attractive for its rapid and large-scale fabrication of 1D nanostructures of ternary complex oxides. The synthesis of sodium and calcium niobate nanorods is a twostep process. First, K2Nb8O21 nanowires were prepared by calcination of Nb2O5 powders in molten KCl at 1000 °C for 3 h. Then, the mixture of K2Nb8O21 nanowires and NaCl (or CaCl2) was heated in a tube furnace at 825 or 800 °C for 3 h and allowed to cool naturally to room temperature. The resulting powders were washed several times with distilled water and then dried at room temperature. Figure 1a shows the X-ray diffraction (XRD) pattern of the assynthesized potassium niobate nanowires. All of the diffraction peaks can be assigned to the orthorhombic phase of K2Nb8O21 (JCPDS 31-1098) with lattice parameters of a ) 3.75, b ) 1.25, and c ) 0.396 nm,14 indicating pure phase of the product. The morphology of the synthesized niobate nanowires was examined by scanning electron microscopy (SEM), as shown in Figure 1b. A large quantity of nanowires with diameters of several hundred nanometers was observed. The length of nanowires can reach up to several tens of micrometers, and most nanowires tend to form bundles (Figure S1). Selected area electron diffraction (SAED) patterns taken from a wealth of nanowires indicate that they are single-crystalline in nature. The growth direction of the synthesized K2Nb8O21 nanowires was determined to be its [001] crystallographic direction (Figure S2). A large scale of sodium niobate nanorods was obtained via an ion-exchange reaction between K2Nb8O21 nanowires and molten NaCl salt, as shown in Figure 2a. The synthesized sodium niobate nanorods, with the same diameters of a few hundred nanowires as that of the precursor, and lengths of several micrometers, show † Harbin Institute of Technology. ‡ Georgia Institute of Technology. Figure 1. (a) XRD and (b) SEM image of potassium niobate nanowires. The angles for the standard sample of potassium niobate are shown at the lower part of panel a.