**1. Introduction**

As a wide bandgap semiconductor material with industrially significant applications, titanium dioxide (TiO2) is commonly used in catalysts [1–3], ointments [4–6], paints [7–9], sunscreens [10–12], and toothpaste [13–15]. Intense study has been put into TiO2 materials since Honda and Fujishima [16] discovered the phenomenon that TiO2 can be used for photocatalytic water splitting. This has allowed TiO2 to be used in photoelectrochemical cells [17–19], photovoltaics [20–22], and photocatalysis [23–25]. TiO2 offers several benefits over other semiconductor materials, including its low toxicity, resistance to photocorrosion, abundance on Earth, and chemical and thermal stability [26]. However, due to its significant recombination rate and broad band gap (3.2 eV), poor quantum efficiency as well as inadequate exploitation of visible light during photocatalytic reaction TiO2's applications is severely limited [27]. Therefore, a variety of methods have been used to alter the TiO2 in an effort to narrow the band gap and lengthen the lifetime of photogenerated charge carriers [28]. These methods include co-doping with metal ion/nonmetal ions, coupling TiO2 with a semiconductor with a small band gap, encasing noble metal cores in a TiO2 shell to create metal core@TiO2 shell composite photocatalysts, noble metal deposition, and surface sensitization by organic dyes [29–32]. Recent research has shown that the defect disorder of TiO2 may influence several of its physical and chemical characteristics, including selectivity, photocatalytic reactivity, and light absorption, among others [33–35].

One of the most prominent defects observed in TiO2 is oxygen vacancies (V0), which are also considered to be common defects in metal oxides and have been studied extensively by using both experimental and theoretical characterizations [36–38]. The V0 has the potential to function as active sites and adsorption points during heterogeneous catalysis [39–41]. The electrical structure, charge transport, surface properties, and other photocatalytic characteristics of metal oxides based on TiO2 have also been demonstrated to be intimately connected to V0 [42–44]. It is theoretically possible that Ti3+ centers or unpaired electrons (e), which could lead to the creation of donor levels in TiO2's electronic structure, are produced as a result of the production of V0 on TiO2 [45–47]. Additionally, it is thought that V0 alters the recombination rate of electron-hole pairs during photocatalysis, which alters the chemical processes that rely on charge transfer from either hole (h<sup>+</sup> ) or e [48–50]. According to theoretical and experimental findings, the excess e on V0 states impacts the reactivity and surface adsorption of important adsorbates like H2O or O2 on TiO2. In order to take use of their special features for photocatalytic applications, controlled synthesis of V0 incorporating TiO2 is of utmost importance [51–53].

In this instructional chapter, we list the methods for producing TiO2 with V0, go over their characteristics, and touch on some of the uses for photocatalysis. The preparation technique is the main division used to classify the syntheses of TiO2 nanomaterials with V0. The readers may consult the relevant literature for comprehensive directions for each synthesis. In Section 3, the reductive, adsorption, optical, and structural characteristics of the TiO2 nanomaterials containing V0 are discussed. To create highly effective photocatalysts and increase the functional applications of photocatalysis, it is intended that this chapter would be a beneficial resource for engineers who want to create defective semiconductors.
