**2. Synthetic strategies used to obtain black Titania nanostructures**

Black titania nanostructures have attracted extensive interest, and various reductive and oxidative approaches have been established to successfully fabricate the black or coloured titania [9]. A variety of structural and chemical modifications are in practice to impart unique features to black titania, like surface amorphousity, oxygen vacancy/Ti3+, hydroxyl groups and Ti–H bonds. Various strategies have been briefly explained below.

## **2.1 Hydrogen treatment**

To reduce TiO2 nanocrystals using heat and hydrogen is an easy approach to obtaining black titania. Thermal hydrogen treatment changes TiO2 (Ti4+) into other chemical species, such as Ti3+ or any other reduction states. Consequently, their lattice structure and chemical/physical properties also change by changing reduction states. The chemistry involved in this reaction is illustrated in the following scheme.

$$\text{TiO}\_2 + \text{xH}\_2 \overset{\text{\textdegree}}{\rightleftharpoons} \text{TiO}\_2, \text{TiO}\_{2\text{-}\text{x}}, \text{TiO}\_{2\text{-}\text{x}} \text{H}\_{2\text{-}\text{}} \tag{1}$$

The above scheme illustrates that the chemical properties and concentration of starting TiO2 nanomaterials, reaction temperature and time, pressure and concentration of hydrogen gas are the various factors that determine the final product of the reaction. When TiO2 nanomaterials are treated with hydrogen, their final properties and the pathway through which the reaction proceeds will be different. This is because the final properties and the direction of reaction depend upon the conditions of hydrogen treatment. Many other factors that affect the chemical properties of nanomaterials are like morphology, shape, size, crystal facets and vacancies contents [10]. Reaction will be more complicated due to these variables. Preparation of these black TiO2 nanomaterials is achieved normally by different research groups using different synthetic strategies. These alterations in structures lead to variations in their properties and functionalities. These structural and functional developments enable us to tune the structural features of a material and then consequent performances.

Hydrogenated environment used to obtain black titania may vary and it includes simply hydrogen thermal treatment, high-pressure pure hydrogen treatment, ambient or low-pressure pure hydrogen treatment, ambient hydrogen-argon treatment, ambient argon treatment and hydrogen plasma treatment.

Chen et al. synthesized black titania NPs *via* treatment of pure white titania NPs with 20.0-bar pure H2 at 200°C for several days [11]. The precursor white titania NPs were synthesized following a solution-based route using titanium tetraisopropoxide

### **Figure 2.**

*(a) Schematic of the formation of black titania. (b) Digital photographs of white and black titania nanomaterials. High-resolution transmission electron microscopy images of (c) white and (d) black titania NPs, (e) UV-vis spectra of white and black titania NPs. Reprinted with permission from reference [11]. Copyright 2011 AAAS.*

as precursor, Pluronic F127 surfactant, hydrochloric acid, deionized water, and ethanol solvents. **Figure 2** illustrates the schematic of the formation of black titania NPs (**Figure 2a**), along with digital photographs (**Figure 2b**) and electron microscopy images (**Figure 2c** and **d**) of white and black titania NPs. The obtained black titania NPs contained a well crystalline lattice core fenced by a disordered lattice shell from the hydrogen treatment. The amorphous boundary was expected to host the external hydrogen dopant and impart black colour to the hydrogenated titania NPs. The black titania NPs had broadband absorption as compared to corresponding untreated white titania as indicated in the UV-visible absorption spectrum (**Figure 2e**).
