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We investigated the degradation of methylene blue (MB) as an organic dye pollutant upon photocatalytic oxidation of TiO2 nanoparticles under UV-LED (395 nm) light irradiation. Effect of different parameters, including initial concentration of dye and catalyst dosage on the degradation rate of the dye were evaluated. We found that the photonic efficiency of the photocatalytic degradation rate of the dye was determined by the ratio between the initial concentration of the dye and the number of TiO2 nanoparticles in the colloidal solution. The optimum photocatalytic degradation rate was achieved when the TiO2 nanoparticles in the solution are well covered by dye molecules, providing an interpretation that MB–TiO2 molecular interactions play the key role in the photoinduced oxidation and reduction, leading to the photocatalytic degradation. We also demonstrated that the energy activation of the photocatalytic degradation is related to diffusion-controlled reaction, indicating that the photocatalytic degradation of the dyes is diffusion-controlled reaction of free hydroxyl radicals.