Recent Advances in MXene-Based Cocatalysts for Photocatalytic Hydrogen Evolution

While MXenes are promising cocatalysts for photocatalytic hydrogen evolution, their roles are frequently oversimplified as merely conductive electron sinks. This review begins by revisiting the classical electron-sink mechanism of Ti3C2 and utilizes the CdSe/Ti3C2 and Cd0.5Zn0.5S/Ti3C2 systems to demonstrate how intimate interfaces, Fermi-level equilibration, Schottky junctions, and directional electron-transfer pathways synergistically couple charge separation with H* conversion. The discussion is then extended to multimetallic MXenes, using ZnIn2S4/Mo2TiC2 as a model, to highlight how metal-layer composition, local interfacial coordination, work-function differences, and hydrogen-adsorption energetics jointly regulate electron extraction and surface reaction kinetics. Special attention is given to surface terminations, which can reshape interfacial energetics and enable MXenes to transition between electron-sink and hole-mediated roles depending on the interface and working conditions. Furthermore, representative Nb2C quantum-dot and VNbC solid-solution systems illustrate how nanoscale electric fields and multimetallic sites can expand MXene functionality beyond conventional electron extraction. Ultimately, this role-oriented framework advances the development of MXene cocatalysts from simple material integration toward controllable and programmable interfacial functions.