Firstly, when semiconductors are illuminated with light energy equal to or greater than the band gap, electron-hole pairs are generated. These photogenerated electrons and holes then migrate to the surface of the semiconductors. Finally, the photogenerated electrons reduce the adsorbed CO 2 on the catalyst, while the holes contribute to the oxidation of water into oxygen . The size of semiconductor nanoparticles plays a crucial role in their photocatalytic behavior. Koci et al. investigated the influence of particle size on the photocatalytic activity of TiO 2 . Their research encompassed particle sizes ranging from 4.5 to 29 nm for the solid-liquid photocatalytic reduction of CO 2 with water. Among these, TiO 2 nanoparticles with an average size of 14 nm exhibited the highest effectiveness. While smaller particles might be expected to be more effective, their extremely small size changes both the optical and electronic properties. This can lead to an increase in the band gap energy, minimizing the efficiency of light utilization . Furthermore, TiO 2 doped with coinage metals co-catalysts including Pt, Pd, Rh, Au, and Ag showed that the rate of CH 4 formation followed the order of Ag < Au < Rh < Pd < Pt. This observation can be attributed to Pt's superior ability to separate photogenerated electron-hole pairs.