## 3 Photocatalytic Materials for Water Treatment
### Composite and Heterojunction Photocatalysts  
Forming heterojunctions by combining two or more semiconductors with different band structures can significantly improve the photocatalytic performance. These systems facilitate charge carrier separation and broaden the light absorption range and benefit from  
Figure 6: Schematic representation of modified and doped photocatalysts: Band gap engineering using metal (Fe, Cu, Ag) and non-metal (N, S, C) doping for enhanced visible-light absorption and reduced recombination, and photocatalytic enhancement with noble metal nanoparticles (Ag, Au, Pt) using surface plasmon resonance (SPR) and improved charge separation.  
synergistic effects between the components. Examples include TiO\({}_{2}\)/g-C\({}_{3}\)N\({}_{4}\), ZnO/WO\({}_{3}\), and BiVO\({}_{4}\)/graphene composites [95; 101].  
Carbon-based supports such as graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanotubes (CNTs) are also employed to enhance electron transport and surface area [81].  
Table 4 provides an overview of various composite and heterojunction photocatalysts, emphasizing their compositions, enhancements, and specific applications in water treatment and environmental remediation.  
Further insights into the application of semiconductor heterojunctions for the degradation of pharmaceutical pollutants are provided in Table 5, which compiles representative studies highlighting various material combinations, operational conditions, and observed efficiencies, demonstrating the versatility and effectiveness of these systems in advanced water treatment [102].  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Photocatalyst** & **Composition** & **Key Features** & **Applications** \\ \hline \multirow{4}{*}{Fe\({}_{2}\)O\({}_{3}\)/TiO\({}_{2}\) heterojunction} & \multirow{4}{*}{Iron oxide (Fe\({}_{2}\)O\({}_{3}\)) and titanium dioxide (TiO\({}_{2}\))} & Optimized weight ratio enhances photocatalytic activity. Effective in antibiotic removal. & Photocatalytic degradation of antibiotics in wastewater. \\ \hline \multirow{4}{*}{g-C\({}_{3}\)N\({}_{4}\)/TNP composite} & \multirow{4}{*}{Graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\)) and TNP-based perovskite materials} & Formation of p–n junction enhances charge separation. Increased photocatalytic activity and stability. & Photocatalytic degradation of organic pollutants in wastewater. \\ \hline \multirow{4}{*}{BiVO\({}_{4}\)@ZIF\(-\)8 composite} & \multirow{4}{*}{Bismuth vanadate (BiVO\({}_{4}\)) and zeolitic imidazolate framework-8 (ZIF\(-\)8)} & High efficiency in photocatalytic wastewater treatment. Enhanced light absorption and charge transfer. & Photocatalytic degradation of pollutants in wastewater. \\ \hline \multirow{4}{*}{WO\({}_{3}\)/Ti-WO\({}_{\mathrm{x}}\)/TiH\({}_{\mathrm{Y}}\)O\(\mathrm{z}\) heterojunction} & \multirow{4}{*}{Tungsten trioxide (WO\({}_{3}\)) and titanium-based compounds} & Extended light absorption above 400 nm. Improved charge separation. & Photocatalytic degradation of azo dye water pollutants under visible light. \\ \hline \multirow{4}{*}{MoS\({}_{2}\)/Ag\({}_{2}\)Mo\({}_{2}\)O\({}_{7}\) composite} & \multirow{4}{*}{Molybdenum disulfide (MoS\({}_{2}\)) and silver} & S-scheme heterojunction improves electron–hole separation. & Photocatalytic degradation of organic pollutants in wastewater. \\ \cline{1-1} \cline{3-3}  & & & \\ \cline{1-1} \cline{3-3}  & & & \\ \cline{1-1} \cline{3-3}  & & & \\ \cline{1-1} \cline{3-3}  & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of composite and heterojunction photocatalysts.