Photocatalytic degradation of hydroxychloroquine with a multifunctional Cucurbit[7]uril based double transition MXene (TiNbCT_x@α-Bi₂O₃/ZnSe) S-scheme heterostructure

In response to the pressing need for efficient, low-energy materials for water purification, we report the design and characterisation of a novel quaternary S-scheme heterojunction photocatalyst: CB[7]-TiNbCT_x@α-Bi₂O₃/ZnSe (CTBZ). This hybrid material integrates α-Bi₂O₃, ZnSe, a double-transition-metal MXene (TiNbCT_x), and cucurbit[7]uril (CB[7]) to enhance visible-light-driven photocatalytic performance. Structural, morphological, and spectroscopic analyses confirmed successful heterostructure formation with tailored optoelectronic properties. UV–Vis spectroscopy revealed strong visible light absorption, while photoluminescence and photoelectrochemical studies demonstrated efficient charge separation and suppressed electron-hole recombination. CTBZ-7 % exhibited reduced charge transfer resistance (84 Ω) and extended charge carrier lifetimes (299 ms), attributed to the synergistic interaction among the components and the S-scheme charge transfer mechanism. Photocatalytic activity was assessed through the degradation of hydroxychloroquine (HCQ) under visible light. Under optimised conditions (pH 7, 5 ppm HCQ, 20 mg catalyst), CTBZ-7 % achieved 92 % degradation within 50 min. Compared to α-Bi₂O₃ (50 %), ZnSe (51 %), BZ (65 %), TBZ-7 % (80 %), there was a 12 % increase in CTBZ-7 % degradation efficiency. CTBZ-7 % had a kinetic rate constant of 2.2 × 10⁻¹/min, significantly surpassing α-Bi₂O₃, ZnSe, BZ, and TBZ-7 %. CB[7] played a dual role in enhancing pollutant adsorption and facilitating proximity-driven charge transfer. Reactive species trapping confirmed superoxide radicals as dominant contributors to degradation, and LC-MS analysis identified nine lower m/z intermediates amenable to downstream removal. The CTBZ-7 % photocatalyst also exhibited excellent stability across four reuse cycles. These findings underscore the promise of CB[7]-engineered heterostructures for sustainable photocatalysis in wastewater treatment and solar-driven environmental remediation.