Photocatalytic Activity and Reusability of F, Sm3+ Co-Doped TiO2/MWCNTs Hybrid Heterostructure for Efficient Photocatalytic Degradation of Brilliant Black Bis-Azo Dye

Abstract

A global freshwater pollution catastrophe is looming due to pollutants of emerging concern (PECs). Conventional water treatment methods are limited in removing PECs such as pharmaceuticals and dye house effluent from aquatic systems. This study provides an effective potential solution by developing an innovative wastewater treatment method based on solar-light-responsive semiconductor-based photocatalysts. A sol-gel synthesis technique was used to produce Fluorine-Sm3+ co-doped TiO2 (0.6% Sm3+) (FST3) photocatalysts. This was followed by loading multi-walled carbon nanotubes (MWCNTs) in the range of 0.25 to 1 wt% into the FST3 matrix. Solid state UV-visible spectroscopy measurements showed a bathochromic shift into the visible light region after the co-doping of TiO2, whereas XRD analysis confirmed the presence of predominantly anatase polymorphs of TiO2. The FT-IR and EDX results confirmed the presence of the F and Sm3+ dopants in the synthesised photocatalysts. XRD and TEM measurements confirmed that the crystallite sizes of all synthesised photocatalysts ranged from 12–19 nm. The resultant photocatalysts were evaluated for photocatalytic degradation of Brilliant Black BN bis-azo dye in aqueous solution under simulated solar irradiation. FST3 completely degraded the dye after 3 h, with a high apparent rate constant (Ka) value (2.73 × 10−2 min−1). The degree of mineralisation was evaluated using the total organic carbon (TOC) technique, which revealed high TOC removal (82%) after 3 h and complete TOC removal after 4 h. The incorporation of F improved the optical properties and the surface chemistry of TiO2, whereas Sm3+ improved the quantum efficiency and the optical properties. These synergistic effects led to significantly improved photocatalytic efficiency. Furthermore, incorporating MWCNTs into the F and Sm3+ co-doped TiO2 (0.6% Sm3+) improved the reaction kinetics of the FST3, effectively reducing the reaction time by over 30%. Recyclability studies showed that after 5 cycles of use, the FST3/C1 degradation efficiency dropped by 7.1%, whereas TiO2 degradation efficiency dropped by 33.4% after the same number of cycles. Overall, this work demonstrates a sustainable and efficient dye-removal technique.