Unraveling the radical generation mechanisms on single-layered Ti3C2Tx MXene and multi-layered V2CTx MXenes for sonocatalytic oxidation

Abstract

Although MXenes, an emerging class of two-dimensional materials, have advanced as promising heterogeneous sonocatalysts, the intrinsic correlation between their surface chemistry and reactive oxygen species generation remains elusive. This study investigates the sonocatalytic mechanisms of single-layered Ti3C2Tx (SL-Ti3C2) and multi-layered V2CTx (ML-V2C), utilizing probe compounds (e.g., methylene blue (MB), acid blue 80, and benzoic acid (BA)). Both MXenes exhibited significant catalytic activity under ultrasonic irradiation, achieving over 85% MB degradation (initial 10 mg/L) within 60 min with synergistic indices up to 3.2. However, several lines of evidence, including results from electron paramagnetic resonance (EPR) spectroscopy, radical scavenging, and quantitative BA probe tests, revealed fundamentally different oxidation pathways in either MXene system. SL-Ti3C2 primarily facilitated the generation of free hydroxyl radicals (•OH) in the bulk solution. In contrast, ML-V2C operated via a surface-confined mechanism; a notable discrepancy between a relatively weak EPR signal and high BA hydroxylation evidenced the dominance of surface-bound •OH (e.g., V2CTx(≡•OH)). X-ray photoelectron spectroscopy analyses further supported that the catalytic performance of ML-V2C is driven by a V4+/V5+ redox cycle and surface hydroxylation. The effects of ultrasonic operational conditions and water-quality factors, including the initial pH and water matrix constituents (e.g., bicarbonate, chloride, and humic acid), on degradation kinetics were further examined. The catalytic performance of the MXenes was maintained over four cycles and under natural water conditions. MB degradation pathways have also been proposed. Overall, the new insights gained provide a mechanistic understanding of the reactions on MXene surfaces in water. © 2026 Elsevier B.V.