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

## TL;DR

This paper explores how photoexcitation affects the surface stability of MXenes, showing that it can change which surface terminations are most stable and useful for photocatalysis.

## Contribution

The study introduces excited-state Pourbaix diagrams to analyze how photoexcitation alters surface termination stability in MXenes.

## Key findings

- Photoexcitation significantly reshapes the stability of surface terminations in MXenes.
- Aqueous acidic etching-related terminations dominate stability regions across all studied MXenes.
- Zr2C's −O termination is both stable and photoactive, while Sc2C and Y2C require alternative synthesis for optimal photocatalytic performance.

## Abstract

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.