MXenes Surface Termination under Photoexcitation: Insights from Excited-State Pourbaix Diagrams

MXenes have emerged as promising materials for photocatalytic hydrogen production, yet their performance is critically dependent on the specific nature of their surface terminations. While Pourbaix diagrams are routinely used to map surface stability under a certain pH and applied external potential (U), they traditionally neglect the influence of photoexcitation on thermodynamic preference. Here, we construct the singlet (S₀) ground state and the lowest triplet (T₁) excited state Pourbaix diagrams for Sc₂C, Y₂C, and Zr₂C MXenes, which have previously shown promising photoactive properties, to assess how photoexcitation alters surface stability. Our results show that constant photoexcitation can significantly reshape the Pourbaix diagrams, altering the thermodynamically preferred surface terminations and thereby influencing photocatalytic behavior. Across all studied systems, terminations associated with aqueous acidic etching environments (-F, -O, -OH, -H) dominate the stability regions. For Zr₂C, this is advantageous since -O termination is both the most stable and photoactive configuration. In contrast, for Sc₂C and Y₂C, the potentially more active halide and chalcogen terminations are overshadowed by aqueous- and HF-derived groups, suggesting that alternative synthesis routes will be required to stabilize the most photocatalytically favorable terminations.