Structural chemistry-driven MXene selection for engineering MXene/polyoxometalate interfaces for photocatalytic...

Rational design of efficient photocatalysts for solar-driven hydrogen production requires a clear understanding of how cocatalyst structure governs interfacial charge-transfer processes. In this study, Keggin-type phosphotungstic acid (PW) modified Ti₃C₂ and Ti₂C MXenes were systematically compared to elucidate the role of MXene structural chemistry in photocatalytic hydrogen evolution. Owing to differences in carbide-layer thicknesses, surface terminations, and electronic structures, the selected MXenes exhibited distinct interfacial coupling behaviors with polyoxometalate clusters. Comprehensive structural, spectroscopic, optical, and electrochemical characterizations revealed that PW incorporation suppressed charge-carrier recombination, and enhanced surface area. In addition, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) results verified the increased oxygen-vacancy density after PW coupling, which was more pronounced for the Ti₃C₂-based system. These changes promoted interfacial charge transfer and improved photocatalytic activity under visible-light irradiation. The optimized Ti₃C₂/PW (2:1) photocatalyst achieved a hydrogen evolution rate of 2496 ± 138 μmol/g_cat. h which was 1.30 times greater than the pristine Ti₃C₂, while Ti₂C/PW reached up to 4773 ± 248 μmol/g_cat. h depending on composition. Although Ti₂C/PW delivered higher peak activity, Ti₃C₂/PW exhibited more stable and predictable structure-activity behavior and was therefore selected for process optimization via response surface methodology, yielding a maximum rate of 2718 μmol/gcat.h under optimized conditions. Both structural and activity analyses demonstrated that the superior performance of MXene/PW arose from stronger interfacial electronic coupling, higher oxygen-vacancy concentration, and favorable Schottky-type junction formation that accelerated electron transfer and proton-reduction kinetics. This work establishes clear structure-activity relationships in MXene/polyoxometalate systems and highlights the importance of structural chemistry-driven MXene selection for engineering efficient photocatalytic interfaces.