**2.2 Chemical reduction**

Chemical reduction is another route to obtain black titania where the Ti+4 species from the corresponding white titania precursors are reduced into low valent Ti species. The reducing agents involved in chemical reduction may include various natural products obtained from plants such as flavonoids, vitamins, phenolic acids, reducing sugars, polysaccharides, triterpenoids, tannins, and polysaccharides as all these are electron rich in nature. The resulting structure of the obtained materials can be tuned by controlling various reaction conditions. Besides these green reductants, some other chemical reducing agents such as aluminium, Zinc, NaBH4, CaH2 and imidazole have been extensively used to reduce TiO2 to black titania.

A representative example of synthesis of black titania through chemical reduction is given here. Wang et al. reported the use of aluminium (Al) as a reducing agent to prepare black titania NPs in an evacuated two-zone vacuum furnace at elevated temperature. To obtain black titania, precursor white TiO2 and Al were placed in separate zone of a two-zone tube furnace and pressure was set at 0.5 Pa through evacuation (**Figure 3a**). The Al was heated at 800°C, whereas precursor white titania was heated at 500°C for 7–18 hours. Thus, obtained reduced black titania NPs displayed intense absorption in the visible regions [12].
