*2.2.4. Summary of solvothermal synthesis*

The solvothermal method in addition to the advantages offered by hydrothermal method allows the use of higher temperatures. This is sometimes advantageous in producing better

**Figure 13.** SEM images (1) nonhierarchical TiO<sup>2</sup> structures grown at 90°C. (a–c) TEM images of 3D hierarchical urchinlike structures having 1D nanoneedles [35].

crystallinity in the products. **Table 3** gives a summary of the reported work on solvothermal synthesis. The prepared HNSs obtained using this method range in size, from a few micrometers to several nanometers. They possess large surface area up to 94 m<sup>2</sup> g−1. The products have good crystallinity due to high crystallization temperatures offered by possible use of high boiling solvents. Different solvents can provide different chelating effects and hindrance to control morphology of structures. In addition to the type of solvent used, temperature and time control the growth and crystallization of particles during synthesis. These types of HNSs can be used for various photocatalytic applications like solar cells and organic pollutant degradation.
