**1. Introduction**

By virtue of their large-scale applications in the removal of environmental pollutants and photocatalytic water splitting, TiO2-based nanomaterials have fascinated massive interest [1–3]. TiO2 exists in three main crystal phases that is brookite, anatase, and rutile. Electronic bandgaps of all these phases are above 3.0 eV, which is considered to be high bandgap. This limits their optical absorption in the ultraviolet (UV) region of the solar spectrum, which is below 5% of overall solar energy. If TiO2 utilized this UV light very efficiently, its solar activity is still not better. It is the number of electrons and holes of the photocatalysts that determine its photocatalytic activity [4].

Upon appropriate light absorption, TiO2 produces excited electrons in conduction band and excited holes in the valence band. For performing photocatalytic reactions, these excited charges, apart from each other, travel towards the surface. From all the excited charges, few of them are combined and vanished during the charge separation and migration processes. This absorption process leads to the generation of excited charges on the surface. If TiO2 absorbs more light, then more excited charges

**Figure 1.** *Mechanistic illustration of the photocatalysis.*

come on the surface. The process is schematically illustrated in **Figure 1**. Therefore, if we improve the optical absorption properties of TiO2, its whole activity can be increased [5, 6].

During last decades, doping techniques have been extensively employed to make TiO2 colourful for desirable optical absorption [7]. For instance, in the early 1990s, a number of metallic species were employed to substitute the Ti4+ ions in the TiO2 lattice [8]. More efforts lead to the doping of several nonmetals till 2001. Currently, many metal and nonmetal elements have been used to substitute partial Ti4+ and O2 ions in the TiO2 lattice. All the aforementioned efforts lead to improved light absorption by TiO2 and consequent photocatalytic performance.

Recently, black TiO2 has become the focus of the research community because of much-enhanced photocatalytic performance. Materials appear black in colour if it absorbs 100% light from the overall visible light region. From the entire visible light region, if TiO2 absorbs certain percentage evenly, then it will become partially black or grey. If no light absorbs from the overall visible light regions, then it will show white colour. In the other case in which light is not absorbed appropriately, different colours (e.g., green, yellow and brown) will be observed. The focus of the research about titania is to tune their colour from lighter to darker. The properties and performance of the TiO2 nanomaterials are affected by the apparent colour. In the proceeding sections, different synthetic strategies to black titania as well as characteristic features and their properties related applications will be discussed.
