Structural and electronic properties of MXene flakes: from edge effects to bandgap evolution

The structural and electronic properties of MXenes were investigated by means of a finite-system approach using all-electron Density Functional Theory-based calculations. Pristine (M₂C)_n flakes and their O-terminated counterparts (M₂CO₂)_n (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W; 12 ≤ n ≤ 216) were computationally modelled. Surface-like behaviour is lost for n ≤ 90, corresponding to ca. 3 nm wide flakes, where finite-size effects become increasingly relevant. While the flake structure at the core is very similar to that found in extended periodic models, the edges are often deformed due to structural defects, which impact their electronic properties. Pristine M₂C flakes are metallic, while the O-terminated M₂CO₂ counterparts present bandgaps exceeding 1 eV for metals of Groups III and IV when neglecting low-populated gap states near the Fermi level. The alignment of the valence and conduction bands for these systems evolves favourably to nearly include the water splitting half-reactions within the bandgap for the largest flakes. Overall, our results show that Sc, Y, Zr, and Hf O-functionalised MXenes are the best suited for photocatalytic water splitting, obtaining energy gaps within the visible spectrum for several flake sizes, and band alignments closer to water oxidation and hydrogen reduction reactions.