*2.1.4. Summary of hydrothermal synthesis*

To summarize, hydrothermal treatment of various precursors under control parameters of temperature, pressure and duration gives a variety of morphologies in HNSs of Titania. The morphology ranges from hierarchical arrangement of nanoparticles to nanorods producing flower-like structures. Increasing temperature increases crystallization of TiO<sup>2</sup> particles, and by increasing duration of hydrothermal synthesis, better etched morphology was obtained. **Table 2** summarizes reported examples of hydrothermal synthesis of HNSs with respect to precursor used for the synthesis and various properties of obtained HNSs together with the application studied. The HNSs obtained from this method varies in size from 20 nm to 4.5 μm. The surface area of the resultant HNSs ranges from 27 to 170 m<sup>2</sup> g−1. These structures are exploited in various applications including solar cells, photocatalysis and sensors, etc. and have improved respective performance owing to their unique structural properties.
