Tuning MXenes Towards Their Use in Photocatalytic Water Splitting

Finding appropriate photocatalysts for solar-driven water (H₂O) splitting to generate hydrogen (H₂) fuel is a challenging task, particularly when guided by conventional trial-and-error experimental methods. Here, density functional theory (DFT) is used to explore the MXenes photocatalytic properties, an emerging family of two-dimensional (2D) transition metal carbides and nitrides with chemical formula M_n+1X_nT_x, known to be semiconductors when having T_x terminations. More than 4,000 MXene structures have been screened, considering different compositional (M, X, T_x, and n) and structural (stacking and termination position) factors, to find suitable MXenes with a bandgap in the visible region and band edges that align with the water-splitting half-reaction potentials. Results from bandgap analysis show how, in general, MXenes with n = 1 and transition metals from group III present the most cases with bandgap and promising sizes, with C-MXenes being superior to N-MXenes. From band alignment calculations of candidate systems with a bandgap larger than 1.23 eV, the minimum required for a water-splitting process, Sc₂CT₂, Y₂CT₂ (T_x = Cl, Br, S, and Se) and Y₂CI₂ are highlighted as adequate photocatalysts.