## 3 Results and Discussion  
### Morphology and Structural Analysis  
The obtained white powder samples were dispersed by ultrasound treatment and observed by scanning electron microscopy (SEM) for a panoramic view. It was found that they were one-dimensional nanostructures. As shown in Figure 1a, the length was more than 20 \(\upmu\)m and the width varied from tens to hundreds of nanometers. It is clear that the as-prepared nanowires were well-shaped and uniform, with a good aspect ratio and dispersion. Figure 1b shows the EDS spectrum taken from the nanowires, which shows that the nanowires contained Na, Ti and O. This suggested that the nanowires are a kind of alkali sodium titanate. The nanowires samples were further analyzed by TEM, as shown in Figure 1d-g. Figure 1d,e show one nanowire and its corresponding selected area electron diffraction (SAED) pattern, and Figure 1f,g show another nanowire and its corresponding SAED pattern. Based on the SAED analysis, the nanowires were identified as a NTO phase, and all the nanowires grew along the (020) direction. For the Na\({}_{2}\)Ti\({}_{7}\)O\({}_{15}\) phase (space group No. 8, Cm), the schematic atomic structure observed along \(b\) axis is shown in Figure 1c, in which alternating groups of three and four TiO\({}_{6}\) octahedra share corners and edges to form a tunnel structure along the \(b\) axis, and Na+-ions are located within the tunnels [7]. Furthermore, a nano-region was selected on the nanowire sample (see the inset in Figure 2a) and further observed using aberration-corrected TEM. As shown in Figure 2a, the high resolution TEM (HRTEM) image shows that there were some \"stripe\" structures along the growth direction in the nanowires. Figure 2b shows the fast Fourier transform (FFT) pattern corresponding to the selected region (red frame) in Figure 2a, which shows the HRTEM that was observed along the (100) zone axis and the \"stripe\" structures along the \(b\) axis of Na\({}_{2}\)Ti\({}_{7}\)O\({}_{15}\). In addition, some fine stripes between the diffraction spots in the SAED patterns (Figure 1e,g) and the FFT pattern (Figure 2b) reveal that there were some stacking faults parallel to the (001) plane. Further enlarging some areas in Figure 2a,  
Figure 1: Morphology, component and structure of the as-prepared Na\({}_{2}\)Ti\({}_{7}\)O\({}_{15}\) nanowires. (**a**) SEM image; (**b**) EDS spectrum; (**c**) Schematic representation of the structure of Na\({}_{2}\)Ti\({}_{7}\)O\({}_{15}\). (**d,e**) TEM image of one selected nanowire and its corresponding SAED pattern with the incident electron beam direction along (100) zone axis; (**f,g**) TEM image of another selected nanowire and its corresponding SAED pattern observed along the (100) zone axis. Figure 2c shows a clearer HRTEM image of the \"stripe\" structures. It is obvious that the \"stripe\" structures consist of the bright and dark atomic planes parallel to the (001) plane. However, it was difficult to identify the channel structure along \(b\) axis from the projected unit cell of Na\({}_{2}\)Ti\({}_{7}\)O\({}_{15}\) along the (100) direction, because of its complicated atomic structure and the undetermined HRTEM imaging condition. We produced the simulation HRTEM images based on the Na\({}_{2}\)Ti\({}_{7}\)O\({}_{15}\) structures along the (100) observation direction, and obtained a series of simulation images with the defocus varying from \(-600\) to \(600\) A and thickness varying from \(0\)-\(600\) A. By comparing the experimental HRTEM image with the theoretical simulation results, the simulation HRTEM image obtained at the condition of \(-100\) A (defocus) and \(150\) A (sample thickness) was selected to be shown in Figure 2d. This means that the channel structure of the NTO NWs along the \(b\) axis presented a dark contrast at this sample thickness and imaging condition.