**1.1. Hierarchical nanostructures**

Hierarchical nanostructures are composed of 3D self-assembly of primary structure (nanoparticle, nanorod, nanotube or nanosheets) in nanoscale. Recently, materials with hierarchical morphology have attracted great attention as compared to spherical nanoparticles. Many experiments have been done in order to perk up the efficiency of nanomaterials by synthesizing hierarchical structures and enhancing the specific surface area and porosity of these structures [8, 38, 40, 53]. These types of structures show utmost light harvesting due to maximum and efficient scattering (hence absorption) of incident light within [40]. Hierarchical TiO<sup>2</sup> nanostructures provide a significant improvement in properties due to enhanced porosity [54, 55] and many devices can be optimized using HNSs [43]. Hierarchical morphology can enhance the photon absorption capability [8] as compared to spherical nanoparticle as there is increased absorption of light due to scattering. The mesoporosity acts as distribution channels increasing adsorption of visible light sensitizers. It also creates an ideal environment for mass transportation [39] of electrons.

This chapter presents a compilation of different synthesis routes and control measures employed for the synthesis of "hierarchical nanostructures of TiO2." A brief overview of each synthesis route is provided. Investigation on the synthesis parameters and the correlation with the characteristic properties of the products are also discussed.
