Structural and Electronic Properties of MXene Flakes: From Edge Effects to Bandgap Evolution

Abstract

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 its 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 flakes structure at the core is very similar to that found in extended periodic models, the edges are often deformed due to structural defects, which impacts 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 evolve favourably to nearly include the water splitting half-reactions into the bandgap for the largest flakes. Overall, our results show that Sc, Y, Zr, and Hf O-functionalized MXenes are the best suited for photocatalytic water splitting, obtaining energy gaps within the visible spectra for several flake sizes, and band alignments closer to water oxidation and hydrogen reduction reactions.