**3.2 Optical properties**

To generate e-h<sup>+</sup> pairs for surface reactions, heterogeneous photocatalysis depends on the ability of photocatalysts to harvest light energy. However, TiO2 can only absorb UV light because of its broad band gap. Fortunately, defect engineering allows for the manipulation of TiO2's optical properties. As local states are created by V0 below the conduction band edge, the light harvesting ability of TiO2 increases from UV to the visible light range. The V0 states that have been developed may participate in a fresh photoexcitation process. In other words, visible light's energy is used to excite the electron from the valence band to the V0 states, resulting in the usual excitations seen in the visible spectrum. Because of this, V0 are referred to as F centers, which comes from the German word for color, Farbe. Additionally, by interacting with nearby Ti4+, the e remaining in the V0 may create the Ti3+ species [92]. Just below the conduction band, the Ti3+ defects may generate a shallow donor level that might also affect the sensitivity to visible light.

### **3.3 Dissociative adsorption properties**

Understanding the active locations on TiO2 has been aided by research into defects using adsorbing probe molecules. On TiO2 single-crystal surfaces, small molecules including HCOOH, O2, H2O, N2O, H2, and CO have been employed to study the performance of such defective sites [93–95]. Some of these molecules' adsorption properties are discovered to change as a result of defects linked to V0 [96]. During the photocatalytic activity, h<sup>+</sup> and e produced in the crystals of TiO2 under UV light may transfer to the surface and then be transported to the adsorbed species, where they take part in the redox reaction [97]. Even though the microscopic specifics of the mechanism of these e transfer are still not fully known, it is anticipated that the transfer will be more effective if the surface and adsorbate are closely connected, as well as when the adsorbed materials are detached.

## *3.3.1 Oxygen adsorption*

In areas like gas sensing and heterogeneous catalysis, the interaction of oxygen with TiO2 is essential. TiO2 catalyzes a number of photooxidation processes, where molecular oxygen acts as the oxidizing agent [98]. One crucial stage of the photocatalytic reaction in these systems is the adsorption of molecular oxygen on the surface of TiO2 [99]. O2 does not, however, adsorb on a perfectly neutral TiO2 surface [100]. Only when there is sufficient negative charge available to form OdTi bonds can O2 adsorb onto the TiO2 surface; this charge may come from subsurface V0 or photogenerated e, with adsorption energies of 2.52 and 0.94 eV, respectively, as shown in **Figure 6**. Superoxide radical groups may be created simultaneously by the O2 adsorbed on the surface of TiO2 and the free e present on V0 states. Both the charge separation process and the oxidation of organic materials are actively promoted by the production of these radical groups [101].

### **Figure 6.**

*Schematic illustration of photoexcited charge carriers in (a) TiO2 and (b) Pt/TiO2. Reprinted with approval from Ref. [101] Copyright American Chemical Society.*
