Bandgap engineering of MXene compounds for water splitting

MXene compounds, a recently discovered family of 2D materials, have been found to become semiconductors and photoactive when their pristine surfaces are functionalized with an electronegative termination. MXenes may present different compositions and structures, depending on the transition metal (M), the X-element (C or N), the stacking, and the termination position. The present work uses density functional theory calculations to engineer the bandgap of a wide range of MXenes by their structure, composition, and surface termination with oxygen, in order to find suitable materials for water splitting photocatalysis. Results show that pristine MXenes present metallic character in all cases, while adding the O-termination increases their chance of becoming a semiconductor. Group III and IV MXenes turn out to be the most promising bandgap systems for photocatalysis. Concerning the X element, C-MXenes exhibit more cases with a bandgap than N-MXenes, and, generally, with larger values. Factors such as the specific O-termination hollow site and stacking may affect the bandgap under certain circumstances, but such influences are found to be subtle and irregular. Finally, for all the cases presenting a bandgap larger than 1.23 eV, the minimum demanded for the water splitting process, the band alignment with respect to the water splitting half-reaction potentials was studied, obtaining Zr2CO2 as a promising photocatalyst for this process.