**4. Photocatalytic applications of black titania nanostructures**

There are three main steps that are involved in photocatalysis. These are absorption of light in the form of photons, electrons, and holes excitation, separation of charges and migration to the surface of photocatalysts, and then the transfer of charges between the photogenerated carriers and the reactant. Prior to the discovery of hydrogenated black titania NPs, it had been demonstrated that hydrogen thermal treatment of titania could improve its photocatalytic properties. Harris and Schumacher discovered that hydrogen reduction at high temperatures decreased recombination canters and prolonged the lifespan of the holes [30]. Oxygen vacancies, Ti3+ species, and hydroxyl groups produced by this process are probably what contributed to the increased photoactivity. Black titania nanostructures exhibit superior performance towards various photocatalytic applications because of the distinct optical and charge transport features as compared to the pristine white titania. Various photocatalytic applications include photocatalytic removal of contaminations, photocatalytic hydrogen production through photoelectrochemical water splitting, photoelectrochemical sensor and photocatalytic CO2 reduction.

## **4.1 Photocatalytic degradation of environmental pollutants**

It has been estimated that about 20% of dye was wasted while the dyeing process in the textile industry and released as effluent in the water. The release of such a large number of coloured dyes into water causes it to be polluted and become a major source of environmental pollution [31, 32]. This polluted water can be photocatalytically detoxified using high-intensity solar energy. Titania is usually thought to be the best photocatalyst and has the tendency to treat wastewater, but only restricted to the ultraviolet region. However, this limitation can be overcome by using coloured titania.

Chen et al. first time prepared the hydrogenated black titania NPs from white titania. The detailed synthetic strategy involves the treatment of white titania with 20 bar of pure H2 at 200°C for five days [11]. The obtained black titania had core-shell structure in which the core is well crystalline, and shell is amorphous. The muchenhanced photocatalytic performance towards methylene blue (MB) degradation was observed under simulated sunlight. For instance, the complete photodegradation of MB was achieved in a very short period of eight minutes under simulated solar light by using black titania as compared to white titania as indicated in **Figure 8a** and **b**.
