## 3 Photocatalytic Materials for Water Treatment
### Modified and Doped Catalysts
Moreover, surface defects can also increase the adsorption of target pollutants, contributing to higher degradation efficiency [91,92,95]. These structural modifications significantly influence the surface properties and electronic behavior of photocatalysts. For example, the presence of oxygen vacancies introduces localized energy levels within the bandgap, which can act as electron traps that prolong charge carrier lifetimes and reduce recombination. These vacancies also increase surface  
Figure 5: Schematic of semiconductor excitation by band gap illumination leading to the creation of “electrons” in the conduction band and “holes” in the valance band (reused from [PERSON] and [PERSON], 2013 [91], under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/license](https://creativecommons.org/license)]([https://creativecommons.org/license](https://creativecommons.org/license)) s/by/4.0/)). reactivity by creating unsaturated coordination sites that facilitate the adsorption and activation of target pollutants, thus improving degradation efficiency. Moreover, such defects can promote the chemisorption of molecular oxygen and water, aiding the generation of reactive oxygen species (ROS) like hydroxyl radicals (\(\bullet\)OH) and superoxide anions (O\({}_{2}\bullet^{-}\)), which are key to the photocatalytic process. The structural and chemical impacts of these modifications are often confirmed by advanced characterization techniques. X-ray photoelectron spectroscopy (XPS) is widely employed to detect oxygen vacancies, reduced oxidation states (e.g., Ti\({}^{3+}\), Bi\({}^{3+}\)), and changes in surface composition through shifts in binding energies or the appearance of new peaks associated with defect states. For instance, the XPS spectra of reduced TiO\({}_{2}\) or doped BiVO\({}_{4}\) have shown an increase in surface oxygen vacancies and Ti\({}^{3+}\) sites, in direct correlation with the improved photodegradation of contaminants such as antibiotics and azo dyes under visible-light irradiation [92; 95; 96; 97]. Similarly, Raman spectroscopy provides insight into lattice distortion, structural disorder, and phonon confinement effects caused by doping or defect formation. The appearance or broadening of characteristic Raman bands (e.g., Eg or B1g modes in TiO\({}_{2}\)) can indicate increased disorder or defect density. The detection of disordered lattice vibrations and Ti\({}^{3+}\)-induced shifts further supports the presence of active surface defects that enhance the charge separation. In combination, these tools validate the successful implementation of defect engineering strategies and help explaining how surface-level changes translate into enhanced photocatalytic performance [98]. In well-established photocatalysts like TiO\({}_{2}\) and ZnO, the defect engineering, especially through the creation of oxygen vacancies and cationic defects, has proven to be particularly effective in enhancing the photocatalytic performance. In TiO\({}_{2}\), the oxygen vacancies act as shallow electron traps, improving charge separation and extending activity into the visible spectrum. For instance, reduced TiO\({}_{2}\) with Ti\({}^{3+}\) surface states had demonstrated increased