Bandgap Engineering on MXene Compounds by Structure, Composition, and Surface Termination

Abstract

Finding adequate photocatalysts for the generation of hydrogen fuel through photocatalytic water splitting via sunlight is a difficult challenge using the traditional trial-and-error method. Thanks to vast advancements in computational methodologies, such as with Density Functional Theory (DFT), the process of identifying suitable photocatalysts may now be accelerated. The present study focuses on MXene compounds, a relatively recently discovered family of two-dimensional early transition metal carbides and nitrides with chemical formula Mn+1Xn, which have been proposed as potentially photoactive materials. An essential step to evaluate their possible use in sunlight-driven photochemistry is the analysis of their bandstructure. Here, DFT simulations are employed in order to inspect the bandstructure of pristine and O-terminated MXene compounds —with M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and X = C, N— as a function of structure, composition, and oxygen surface termination, with the goal of finding suitable compounds for photocatalytic water splitting. From the studied MXenes and their possible configurations, pristine MXenes have shown metallic character for all cases, while adding the O-termination has proven to increase the chance of becoming a semiconductor, especially for Group III and IV MXenes, which posed as the most promising bandgap cases for photocatalysis. Concerning the X element, C-MXenes exhibit more cases with bandgap than N-MXenes, and generally with larger values. Factors such O-termination hollow position 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 thermodynamically demanded to carry out the water splitting process, the band alignment with respect the water splitting half-reactions potentials was studied. Results highlighted Zr2CO2 and Y2NO2 as, a priori, adequate photocatalysts for this process.