Factors contributing to reduced photocatalytic activity in Cu-loaded TiO₂/activated carbon composites for methylene blue degradation

TiO₂ is a semiconductor widely used in photocatalytic applications due to its excellent chemical stability, yet it has a low surface area. Incorporating TiO₂ on the surface of activated carbon via a hydrothermal process could enlarge its surface area, and therefore the photocatalytic activity. Theoretically, metal deposition on the surface of TiO₂ could create a Schottky junction, which suppresses charge recombination and enhances photocatalytic activity. Herein, we report Cu deposition on the surface of TiO₂ anchored on activated carbon (Cu-TiO₂/AC) for photocatalytic degradation of methylene blue. The Cu was deposited by a photodeposition process under UV irradiation. In contrast to the conventional hypothesis, that the Cu deposition on TiO₂/AC could exhibit an enhanced photocatalytic activity, our current result shows that the material exhibits a decrease in photocatalytic activity compared to the pristine TiO₂/AC. The same trend was also observed in other TiO₂ samples with different morphology upon Cu photodeposition. The decrease in photocatalytic activity could be correlated with the size of Cu nanoparticles and the oxidation state of Cu species. The photodeposition typically produces a relatively large Cu nanoparticle, which may serve as the recombination center for the charge carriers. Moreover, the existence of Cu⁺ and Cu²⁺ species, in addition to Cu⁰, could consume the transferred e^- from TiO₂, decreasing the e^- efficiency. This work implies that ensuring the small particle size and the existence of monovalent Cu⁰ species are critical factors for using Cu as a co-catalyst in photocatalyst materials.