Hydrothermal synthesis has been an interesting technique to prepare materials with different nanoarchitectures such as nanowires, nanorods, nanobelts, nanourchins, and so forth. The main advantage of the hydrothermal technique over other soft chemical routes is the ability to control the nanostructures ranging from nanoparticles to nanorods or nanourchins to nanotubes by properly choosing the temperature or time of the reaction or the active fill level in the pressure vessel or solvent used for the reaction without any major structure-directing agents or templates.

Beside hydrothermal synthesis, sol–gel method is a simply and low temperature and pressure method for the synthesis of nanostructured metal oxides. {Hashemzadeh, F., Mehdi Kashani Motlagh, M., & Maghsoudipour, A. (2009). A comparative study of hydrothermal and sol–gel methods in the synthesis of MnO2 nanostructures. Journal of Sol-Gel Science and Technology, 51(2), 169–174.doi:10.1007/s10971-009-1978-2 }