## 2 Core Concepts of Heterogeneous Photocatalysis
### Factors Affecting Photocatalysis Efficiency  
The overall performance of photocatalytic systems is influenced by several interconnected factors that determine their efficiency and applicability in environmental remediation [60; 61; 62; 63; 64]. These key factors are summarized in Figure 4.  
Understanding and optimizing these factors are essential to improving the degradation efficiency and sustainability of the photocatalytic treatments. Figure 4: Key parameters influencing the efficiency of photocatalytic degradation processes, including light source and intensity, charge carrier recombination, catalyst surface properties, solution chemistry, and catalyst dosage. These factors collectively determine the effectiveness and reliability of photocatalytic water treatment systems. Figure 3: The fundamental mechanism of photocatalytic degradation of contaminants in water under light irradiation (reused from [PERSON] et al., 2023 [PERSON] et al. (2023), under the CC-BY-Non-Commercial (NC)-No-Derivatives (ND) license where users can download and distribute the work without any alterations of any kind, after giving appropriate credit to original source). UV light (especially UV-A) is commonly used with materials like TiO\({}_{2}\) due to its strong photocatalytic activity. UV-A radiation, which is most commonly used in photocatalysis, typically spans the wavelength range of 315-400 nm, while UV-B ranges from 280 to 315 nm and UV-C from 100 to 280 nm. These distinctions are important when evaluating the activation thresholds of different photocatalysts, especially those responsive to near-visible light. * Visible-light-responsive photocatalysts (e.g., doped TiO\({}_{2}\), g-C\({}_{3}\)N\({}_{4}\), ZnFe\({}_{2}\)O\({}_{4}\)) (wavelength range of 400-700 nm) are gaining popularity for solar-driven applications, enhancing environmental compatibility and energy efficiency. * Both light intensity and irradiation duration directly affect the rate of electron-hole pair generation. Higher light intensity increases, in principle, the number of incident photons on the photocatalyst surface per unit time, thereby enhancing the excitation of electrons from the valence band to the conduction band and increasing the generation rate of electron-hole pairs. Similarly, prolonged irradiation duration extends the cumulative exposure, enabling more excitation events over time. However, excessively high intensity or duration may also lead to recombination or thermal effects that reduce the overall efficiency. The light source is a critical component in photocatalytic systems, directly influencing the degradation efficiency. Key aspects such as illumination intensity, emission spectrum, and source design and positioning affect the activation of the photocatalyst [66; 67]. In setups using artificial light, uniform photon distribution and energy-efficient operation must be ensured. The light must penetrate the reaction medium effectively, despite potential scattering, and its spectral output should match the photocatalyst's band gap to initiate the electron-hole pair generation [68; 69]. (ii) Recombination of charge carriers  
A major limitation in photocatalysis is the rapid recombination of photogenerated electrons (e\({}^{-}\)) and holes (h\({}^{+}\)), which reduces the availability of reactive oxygen species (ROS) such as hydroxyl radicals (\(\bullet\)OH). Once excited by light, the electrons may quickly recombine with holes if they are not effectively separated or transferred. This recombination suppresses photocatalytic activity and lowers overall efficiency [70; 71].