Schematic illustration of modified g-C ₃ N ₄ for visible-light photocatalytic water splitting to hydrogen. The CN-DPT shows remarkably enhanced hydrogen evolution performance.

The dye wastewater produced in the printing and dyeing industry causes serious harm to the natural environment. TiO₂ usually shows photocatalytic degradation of dye under the irradiation ultravilet light rather than visible light. In this work, a large number of oxygen vacancies and Ti³⁺ defects were generated on the surface of the TiO₂ nanoparticles via Ar plasma. Compared with pristine TiO₂ nanoparticles, the as-obtained Ar plasma-treated TiO₂ (Ar-TiO₂) nanoparticles make the energy band gap reduce from 3.21 eV to 3.17 eV and exhibit enhanced photocatalytic degradation of organic dyes. The Ar-TiO₂ obtained exhibited excellent degradation properties of methyl orange (MO); the degradation rate under sunlight irradiation was 99.6% in 30 min, and the photocatalytic performance was about twice that of the original TiO₂ nanoparticles (49%). The degradation rate under visible light (λ > 400 nm) irradiation was 89% in 150 min, and the photocatalytic performance of the Ar-TiO₂ was approaching ~4 times higher than that of the original TiO₂ nanoparticles (23%). Ar-TiO₂ also showed good degradation performance in degrading rhodamine B (Rho B) and methylene blue (MB). We believe that this plasma strategy provides a new method for improving the photocatalytic activity of other metal oxides.