## 3 Photocatalytic Materials for Water Treatment
### Modified and Doped Catalysts  
Band gap engineering using metal and non-metal doping has proven effective in enhancing visible-light absorption and reducing recombination. Bandgap engineering refers to strategies used to modify the energy band structure of photocatalytic semiconductors to enhance their light absorption, especially in the visible range. Traditional materials like TiO\({}_{2}\) and ZnO have wide bandgaps that mostly restrict activity to UV light [14; 31]. To overcome this, several approaches have been employed. Doping with metal or non-metal ions (e.g., N, S, Fe, Cu) introduces intermediate energy states within the bandgap, facilitating visible-light activation [49; 51; 60]. Heterojunction formation between semiconductors with different band structures improves the charge separation and extends the absorption range [65; 69]. Sensitization with dyes or narrow-bandgap semiconductors, as well as cou \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \multicolumn{2}{p{56.9 pt}}{**Photocatalyst**} & \multicolumn{1}{p{113.8 pt}}{**Band Gap Energy (eV)**} & \multicolumn{1}{p{113.8 pt}}{**Key Features**} & \multicolumn{1}{p{113.8 pt}}{**Applications**} \\ \hline \multirow{3}{*}{WO\({}_{3}\)} & \multirow{3}{*}{2.5–2.8} & Effective under visible light. & \multirow{3}{*}{\begin{tabular}{} \end{tabular} } \\  & & High stability. & \\  & & Suitable for environmental remediation. & \\ \hline \multirow{3}{*}{g-C\({}_{3}\)N\({}_{4}\)} & \multirow{3}{*}{\(\sim\)2.7} & Metal-free. & \multirow{3}{*}{\begin{tabular}{} \end{tabular} } \\  & & Strong visible light absorption. & \\  & & Tunable electronic structure. & \\ \hline BiVO\({}_{4}\) & 2.4–2.5 & \multirow{3}{*}{\begin{tabular}{} \end{tabular} } \\  & & Strong visible light absorption. & \\  & High stability. & \\ \hline CdS & \(\sim\)2.4 & \
\begin{tabular}{} \end{tabular} \\  & & Narrow band gap. & \\  & Effective visible light absorption. & \\  & Prone to photocorrosion. & \\ \hline ZnO/g-C\({}_{3}\)N\({}_{4}\) &
\begin{tabular}{} \end{tabular} \\  & & Enhanced visible light absorption. & \\  & composition & Improved charge separation. & \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of visible-light-responsive photocatalysts. pling with plasmonic nanoparticles, are also effective in tuning optical properties [70,71,73]. These bandgap engineering strategies are central to the development of high-performance photocatalysts that are active under visible light. The main bandgap engineering strategies used to enhance visible-light-driven photocatalysis include non-metal doping, metal doping, and the integration of noble metal nanoparticles. * Non-metal doping: Elements such as nitrogen, sulfur, or carbon are commonly used; they can narrow the band gap and improve the visible-light activity. * Metal doping: Iron, copper, and silver serve to modify the electronic structure or act as electron traps [75,92]. They can introduce new energy levels in the solid and enhance the charge transfer. * Noble metal nanoparticles (e.g., Ag, Au, Pt): These nanoparticles can enhance photocatalysis via surface plasmon resonance (SPR), which increases light absorption and promotes charge separation when integrated with semiconductors, boosting photocatalytic efficiency [93,94]. Table 3 provides an overview of various modified and doped photocatalysts, emphasizing their enhancements and specific applications in water treatment and environmental remediation. Defect engineering, including the intentional introduction of oxygen vacancies or surface defects, is another effective strategy to improve the photocatalytic performance. These defects can introduce localized states within the bandgap, enhance visible-light absorption, and serve as active sites for redox reactions. In materials like TiO\({}_{2}\), ZnO, and BiVO\({}_{4}\), oxygen vacancies have been shown to improve charge separation and reduce recombination rates by acting as shallow traps for the photoinduced carriers.