Engineering Charge Transfer of MXene-Hybridized Photocatalysts: An In Situ Kelvin Probe Atomic Force Microscopy Study

Functioning as nanoscale "conductors", 2D transition metal carbides (MXenes) orchestrate interfacial charge-carrier transport between semiconductors. This capability enables precision-engineered control over photocatalytic processes by regulating charge-carrier separation and migration dynamics. However, existing techniques inadequately resolve photogenerated carrier dynamics, hindering rational MXene photocatalyst design. This study addresses this limitation using in situ nanoscale imaging with Kelvin probe force microscopy (KPFM) on a model TiO₂@MXenes system. We demonstrate that the charge transfer direction is governed by work function (W_F) difference, allowing for utilizing MXenes as electron- or hole-accepter by tuning the work function of MXenes. Glycerol treatment increases surface ─OH density, lowering MXene W_F to 1.31 eV and strengthening hole-extracting electric fields. By doing so, Pt/TiO₂ modified with low-W_F MXenes, showing 70.7%-91.7% reduction in charge transfer resistance, displayed the electron-hole transport, which vastly outperforms the electron-electron transport mode. This work offers a quantitative design paradigm linking surface terminations to W_F to carrier transport pathways for developing efficient MXene-based heterojunction photocatalysts.