Plasma-Treated TiO ₂ Nanostructures for Photocatalysis under Ultraviolet A and Visible Light Irradiation

Sustainable methods are needed to remove organic compounds from nuclear waste streams and reduce reliance on energy-intensive processes such as incineration. Visible-light photocatalysis, an emerging advanced oxidation process, offers a low-energy, low-waste approach for degrading toxic organic pollutants and shows strong potential for treating organics in nuclear waste. Visible-light photodegradation requires efficient photocatalysts. Herein, plasma-treated (PT) TiO2 nanostructures (PT-TiO2 NS) with visible-light activity were synthesized via a one-step hydrothermal method. In this process, TiO2 nanoparticles (TiO2 NP; average particle size = 22.4 nm) were first subjected to alkaline treatment with a 5 M NaOH aqueous solution, followed by the treatment of the obtained nanostructures (TiO2 NS) with an Ar-H2 (1.5% vol H2) nonequilibrium plasma. For comparison, TiO2 NP were also directly treated with an Ar-H2 (1.5% vol H2) plasma to obtain PT-TiO2 NP. The photocatalytic activity of these materials was evaluated by assessing their performance in the degradation of organic compounds commonly found in nuclear waste streams, such as oxalic acid and citric acid, under ultraviolet A (UVA) and visible light-emitting diode (LED) irradiation. The degradation products were analyzed using nuclear magnetic resonance spectroscopy. Transmission electron microscopy revealed a mixture of nanostructures, including nanotubes, nanosheets, and nanorods, in the TiO2 NS photocatalyst. X-ray photoelectron spectroscopy confirmed changes in the surface chemical bonding of PT-TiO2 NP and PT-TiO2 NS induced by the Ar-H2 (1.5% vol H2) plasma treatment. Under visible LED irradiation for 72 h, oxalic acid photodegradation (%) followed the order of PT-TiO2 NS (78%) > TiO2 NS (60%) > PT-TiO2 NP (46%) > TiO2 NP (29%). Citric acid photodegradation achieved with selected photocatalysts followed the order of PT-TiO2 NS (100%) > TiO2 NP (26%) under visible LED irradiation for ∼160 h. Among all the catalysts, PT-TiO2 NS exhibited the highest photocatalytic activity under both UVA and visible light.