## 3 Results and Discussion
### Effects of Reaction Temperature on SnO\({}_{2}\) Morphology  
The effects of the reaction temperature on SnO\({}_{2}\) morphology were investigated with 50 mg of PVP, a reaction time of 12 h, and V = 1:1. As observed in Figure 4b-f, SnO\({}_{2}\) morphology is dominated by lamelliform structures at high reaction temperatures. Meanwhile, microelements of 1 DSnO\({}_{2}\) nucleate at multiple sites on the flakes and grow via aggregation induced by the reaction conditions, thereby forming the microsphere structures of 1 DSnO\({}_{2}\) self-assembly. More such microspheres are formed as the reaction temperature increases. At 200 \({}^{\circ}\)C, the lamelliform morphology disappears and is replaced by the microspheres of 1 DSnO\({}_{2}\) self-assembly. Based on the morphology and structures, 1 DSnO\({}_{2}\) is formed at the reaction temperature of 200 \({}^{\circ}\)C.  
Only 1 DSnO\({}_{2}\) were observed in Figure 4a. So, the optimum experimental conditions were at the reaction temperature of 200 \({}^{\circ}\)C, reaction time of 12 h, and V = 1:1. With the addition of 50 mg PVP, the product morphology exhibited microsphere structures of 1 DSnO\({}_{2}\) self-assembly. The composition and microstructure of 1 DSnO\({}_{2}\) nanocrystals are significantly affected by the reaction temperature. Below 200 \({}^{\circ}\)C, the condensation degree among Na\({}_{2}\)Sn(OH)\({}_{6}\) mesophases is low, which may significantly decelerate the growth of 1 DSnO\({}_{2}\). However, by raising the temperature to 200 \({}^{\circ}\)C and reacting for 12 h, 100% 1 DSnO\({}_{2}\) can be formed. The conversion of Na\({}_{2}\)Sn(OH)\({}_{6}\) into tin oxide is more apparent at a higher temperature, and the grain size of samples increases slightly upon increasing the hydrothermal temperature. This is because the pressure inside the reactor increases with the hydrothermal temperature and provides sufficient energy for grain growth. During the process, small grains are dissolved to increase the solute concentration, thereby facilitating the growth of large grains, which conforms to the mechanism of Ostwald ripening [22]. Ostwald ripening occurs via the dissolution and re-precipitation of matter at regions with small and large radii of curvatures, respectively. Ostwald ripening results in the  
Figure 3: SnO\({}_{2}\) morphology at 200 \({}^{\circ}\)C, with 50 mg PVP, V = 1:1, and reacting for (**a**) 48, (**b**) 36, (**c**) 30, (**d**) 24, (**e**) 18, and (**f**) 12 h.  
dissolution of smaller solid grains, the diffusion of the solute throughout the liquid, and the re-precipitation of the solid into large grains. The net result is grain growth. The XRD, TEM, and SEM results of 1 DSnO\({}_{2}\), prepared at the condition, are stated as follows. The diffraction peaks of the products appear sharp and free of impurity peaks, indicating superior crystallinity and high purity of the final products. Here, (110), (101), and (211) are the three relatively strong peaks. Compared to the standard card, the results are consistent with those of the tetragonal rutile SnO\({}_{2}\) (JCPDS No. 41-1445, S.G.:P42/nm, a = b = 0.4738 nm, c = 0.3187 nm). Contrary to the standard card, the peak intensities of (101), (211), and (002) in the sample are significantly enhanced. According to [PERSON] et al. [6], peak (002) is the characteristic peak of SnO\({}_{2}\)(NR) array, and its enhancement suggests an improvement in the degree of order in the sample. The crystal structure is demonstrated in Figure 5a. Each Sn coordinate contains six oxygen atoms at the vertices of the twisted octahedron. Among these, four oxygen atoms are in the same plane, and the Sn-O bonds are shorter than those of the other two oxygen atoms.