## 3 Photocatalytic Materials for Water Treatment
### Modified and Doped Catalysts
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Photocatalyst** & **Modification/Doping** & **Key Features** & **Applications** \\ \hline \multirow{4}{*}{N-TiO\({}_{2}\)} & \multirow{4}{*}{Nitrogen doping} & Enhanced visible light absorption. & Photocatalytic degradation of organic pollutants. \\  & & Improved photocatalytic activity. & & \\ \hline \multirow{2}{*}{Ag@N-TiO\({}_{2}\)} & Silver and nitrogen doping & Enhanced visible light absorption. & Photocatalytic inactivation of pathogenic bacteria in wastewater treatment. \\  & & Antibacterial properties. & & \\ \hline \multirow{2}{*}{ZrO\({}_{2}\) doped with noble metals} & \multirow{2}{*}{Noble metal doping} & Tuned bandgap energy. & Photocatalytic degradation of organic pollutants. \\  & & Enhanced photocatalytic activity. & & \\ \hline \multirow{2}{*}{WO\({}_{3}\)/Ti-WO\({}_{\chi}\)/Ti\({}_{\gamma}\)O\(\underline{z}\)} & \multirow{2}{*}{Homo/heterojunction} & Extended light absorption above 400 nm. & Photocatalytic degradation of azo dye water pollutants \\  & & Improved charge separation. & under visible light. \\ \hline \multirow{2}{*}{ZnO/NiFe\({}_{2}\)O\({}_{4}\) nanocomposite} & \multirow{2}{*}{Composite formation} & High degradation efficiency under UV light. & Photocatalytic degradation of organic dyes in wastewater. \\  & & Effective against methylene dye. & \\ \hline \multirow{2}{*}{MgO/graphene nanoplatelets} & \multirow{2}{*}{Composite formation} & Significant photocatalytic activity. & Purification of industrial wastewater. \\  & & Enhanced antibacterial performance. & Potential applications in nanomedicine. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of modified and doped photocatalysts. degradation efficiency for pollutants such as methyl orange and tetracycline under visible light [91; 95]. Similarly, ZnO is prone to intrinsic defects such as zinc interstitials and oxygen vacancies, which can modulate the band structure and facilitate light absorption beyond the UV range. Studies have shown that ZnO with controlled defect concentration exhibits improved photodegradation rates of dyes and antibiotics, attributed to enhanced ROS generation and pollutant adsorption [86; 99; 100]. These defect-based modifications are typically confirmed using XPS and photoluminescence (PL) spectroscopy, which provide insight into trap states and recombination dynamics. As such, defect engineering presents a cost-effective and scalable route to improve the efficiency and applicability of TiO\({}_{2}\)- and ZnO-based photocatalysts. Figure 6 illustrates the role of modified and doped catalysts in enhancing photocatalytic performance. It emphasizes mechanisms such as band gap narrowing, improved charge separation and surface plasmon resonance. The left side of Figure 6 shows a semiconductor material modified by non-metal dopants like nitrogen, sulfur, and carbon, which integrate into the crystal lattice. This doping narrows the band gap, enabling the material to absorb visible light more effectively. Metal dopants such as iron, copper, and silver are also shown, and these can either introduce new energy levels within the band gap or act as electron traps, reducing the electron-hole recombination. On the right, noble metal nanoparticles (Ag, Au, Pt) are depicted on the semiconductor surface. They enhance the photocatalytic activity via surface plasmon resonance (SPR), where collective oscillations of conduction electrons under light exposure increase the local electromagnetic fields, boosting light absorption and promoting an efficient charge carrier separation. Arrows indicate the movement of electrons and holes, illustrating the reduced recombination and enhanced photocatalytic efficiency under visible-light irradiation.