## 2 Core Concepts of Heterogeneous Photocatalysis
### Factors Affecting Photocatalysis Efficiency
Strategies to suppress recombination and facilitate charge separation and prolong carrier lifetimes include the following [72; 73]:  
- Heterojunction constructions (e.g., Z-scheme or type-II)  
- Noble metal deposition (doping) (e.g., Ag, Au) as electron sinks  
- Co-catalyst integration (e.g., reduced graphene oxide, carbon quantum dots)  
(iii) Surface properties of the photocatalyst  
The surface characteristics of a photocatalyst are essential for its reactivity and pollutant degradation performance. The surface characteristics of a photocatalyst directly impact its interaction with contaminants [74; 75; 76]. * High specific surface area increases the number of active sites. * Porosity and surface functional groups (e.g., hydroxyl or amine groups) promote adsorption and improve photocatalytic kinetics. * Morphology (nanorods, nanosheets, hollow spheres) and particle size also influences light absorption and diffusion behaviors. * Solution parameters  
The efficiency of the photocatalytic processes is not determined by the catalyst alone; the composition of the surrounding aqueous environment also plays a crucial role. Severalsolution-related factors can alter the surface interactions, photon absorption, and the generation of reactive species, ultimately impacting the overall degradation performance. The chemical composition of the aqueous medium can significantly the influence photocatalytic activity [77; 78; 79]:  
* The pH affects the surface charge of the photocatalyst and the ionization state of pollutants. * The ionic strength and the presence of natural organic matter (NOM) or inorganic ions (e.g., Cl\({}^{-}\), HCO\({}_{3}\)\({}^{-}\)) can compete for active sites or scavenge the ROS. * Turbidity and colored substances may interfere with light penetration. Catalyst loading as an efficiency-determining parameter. The quantity of photocatalyst used in a reaction system is a key parameter that directly influences the photocatalytic efficiency. Achieving an optimal catalyst loading is essential to ensure sufficient active sites for pollutant degradation, while maintaining effective light utilization and mass transfer. Both underloading and overloading can adversely affect performance, making careful optimization necessary based on the specific system configuration and contaminant characteristics. An optimal amount of photocatalyst is necessary to balance the efficient photon absorption with effective mass transfer [80; 81]. * Low solid loading limits the degradation due to insufficient active sites. * High solid loading can cause light scattering or shielding effects, decreasing the photon penetration and reducing efficiency. * The optimum concentration depends on the reactor design, contaminant type, detailed composition of the aqueous mixture, and catalyst nature. One major limitation of current photocatalytic systems is the fast recombination of photogenerated electron-hole pairs, which significantly reduces quantum efficiency. While many strategies, such as doping, heterojunction formation, and plasmonic enhancement are proposed to overcome this, their effectiveness is often inconsistent across different experimental setups. Some studies report up to a fivefold increase in degradation rate using heterostructures, such as Z-scheme TiO\({}_{2}\)/g-C\({}_{3}\)N\({}_{4}\) and BiVO\({}_{4}\)/graphene composites, due to the improved charge carrier migration, as presented by [PERSON] et al. [71] and [PERSON] et al. [72]. However, other investigations have failed to reproduce these enhancements consistently, attributing the limitations to interfacial charge trapping, instability of heterojunctions, or synthesis variability under real water matrices [74; 75]. These discrepancies underscore the need for more standardized testing conditions and reproducibility studies to guide the rational design of robust photocatalytic systems.