**3. Characteristic features of black Titania nanostructures**

It has been frequently stated that the various black titania nanostructures showed distinct chemical and physical characteristics, as briefly explained in the following sections, since the fabrication techniques and synthesis conditions of black titania nanostructures differ from one another in the literature. The observed black colour of the titania NPs has been attributed to some of these characteristics. The unique features responsible to impart colour to titania are given below.

### **3.1 Structural disorderness near the surface**

According to certain investigations, the surface of black titania nanostructures with a crystalline/disordered core-shell structure shows disordered structural features. However, literature also contains different reports. For instance, Chen et al. reported the existence of a disordered surface layer surrounding the crystalline core in hydrogenated black titania NPs obtained in the conditions of 20 bar hydrogen pressure and 200°C [11]; Lu et al. have observed that hydrogenated titania nanocrystals made by treating commercial Degussa P25 under 35 bar hydrogen pressure and room temperature for up to 20 days also contain disorderness in the structure near surface [17]. Wang and Xu, observed the same structural features in the hydrogenated black titania nanosheets [18]. Therefore, the hydrogenation treatment also suggested a modest lattice expansion. Some groups reported lattice shrinkage in the disordered layer [19]. The surface of the hydrogenated titania NTs, on the other hand, was extremely transparent, according to Lu and Zhou et al. [20].

The disordered phase of the black titania nanostructures may be distinguished from the crystalline phase using high-resolution transmission electron microscopy (HRTEM). For instance, **Figure 6** illustrates the comparison of the structure of black and white titania NPs using HRTEM and line analysis [21]. Even at the surface of the nanocrystal, the white titania nanoparticle revealed sharply defined, and well-resolved lattice fringes (**Figure 7a**), and the spacing between the adjacent lattice planes was uniform throughout the whole nanocrystal and typical for anatase (0.352 nm) (**Figure 7b**). The crystalline-disordered core-shell structure of the black titania nanoparticle (**Figure 7c**) revealed a structural divergence from the typical crystalline anatase at the outer layer, where the straight lattice line was curved at the nanoparticle's edge, and the plane distance was no longer uniform (**Figure 7d**). The distinction between the amorphous structure and the crystalline phases has occasionally also been made using electron diffraction (ED).
