## 3 Results and Discussion
### Effects of Reaction Temperature on SnO\({}_{2}\) Morphology
TEM analysis is conducted on the products, as illustrated in Figure 6.  
As verified from the TEM and SEM images, the products displayed a morphology of 100% 1D nanostructures with a length of 422.87 \(\pm\) 110.15 nm, width of 81.4 \(\pm\) 16.75 nm, and aspect ratio of 5:1, indicating the prevalence of strictly anisotropic growth during the entire process. The SEM result in Figure 6a indicates that the front end of a single nanorod resembles a pyramid, which is consistent with the FESEM result illustrated in Figure 6c. The inset portrays that the spacing between adjacent lattice planes corresponding to the [110] plane of rutile tin oxides is 0.34 nm, which indicates that the preferential growth direction is [001]. Projecting outward from the centre of the nanolayers, Figure 6b,c demonstrate the quasi-microspheres with a diameter of 2 um, which are formed by the self-assembly of 1D nanorods. The highly magnified SEM image further indicate that all nanorods resemble a square (Figure 6c). The growth model with thermodynamic stability of square tin-oxide nanowires (SnO\({}_{2}\)(NW)) in the solution is demonstrated in Figure 6f. The square tin-oxide  
Figure 4: SnO\({}_{2}\) morphology with 50 mg PVP, reacting for 12 h, and V = 1:1: (a) 200, (b) 190, (c) 180, (d) 170, (e) 160, and (f) 150 \({}^{\circ}\)C.  
nanorods grow from the centre of the quasi-microspheres, then elongate and protrude along the \(c\)-axis, thereby forming layered nanostructures. Figures 6 and 7 show the different morphological types at different sites of same product. Figure 7 shows the morphologies of SnO\({}_{2}\) nanowires. These nanowires are estimated to be about 200 \(\pm\) 45.24 nm in length and 15 \(\pm\) 5.62 nm in width, and the aspect ratio of the nanowires is up to 1:13. Figure 5: **(a)** Crystal structure and (**b**) XRD pattern of (1D)SnO\({}_{2}\) prepared via the developed method at 200 \({}^{\circ}\)C for 12 h.  
Figure 6: TEM and FESEM images of SnO\({}_{2}\)(NR): (**a**) low-magnification TEM image of single SnO\({}_{2}\)(NR), (inset) calibration of Fourier transform mode and interplanar crystal spacing; (**b**) 1 \(\mu\)m FESEM image; (**c**) 500 nm FESEM image. Frequency statistics charts of (**d**) nanorod length and (**e**) nanorod width. (**f**) Growth model of synthesised SnO\({}_{2}\) nanorods with thermodynamic stability. The specific surface energy of crystalline SnO\({}_{2}\) has been calculated earlier [23,24]. The same tendency is demonstrated in the results, i.e., the crystal surface energy increased in the order of (110) \(<\) (100) \(<\) (101) \(<\) (001) with incremental energy. As the lowest and highest surface energies appear on (110) and (001) surfaces, the growth in the [001] orientation yielded a higher aspect ratio of the particles than that in other orientations. The experimental results of SnO\({}_{2}\)(NR) obtained in this study are consistent with the calculation results, as single crystal nanorods grow along the [001] orientation of the long axis. Thus, 1 DSnO\({}_{2}\) was prepared, adopting the hydrothermal method using SnCl\({}_{4}\) -5H\({}_{2}\)O as the tin source, absolute alcohol and deionised water as the solvent, and PVP as the surfactant. Our hypothesis of the formation process is stated as follows. At the initial stage of the hydrothermal reaction, Na\({}_{2}\)Sn(OH)\({}_{6}\) colloids are formed as condensation nuclei. As the temperature and pressure in the reaction system increases, small particles aggregate into large particles according to the mechanism of Ostwald ripening. The particle morphology evolves into lamelliform structures with temperature, pressure, and prolonged reaction period. As observed in the highly magnified SEM, small protrusions appear on the lamelliform morphology, which grow gradually during the process of hydrothermal crystallisation. The crystals will grow preferentially along the crystal plane with low nucleation energy, i.e., the growth rate of crystals is higher along the direction with lower nucleation energy on the corresponding crystal plane.