Quantum capacitance, structural, optical and electronic properties of Zr2CT2 (T = F, P, Cl, Se, Br, O, S, OH) MXenes: A DFT study

Surface functionalization with termination functional groups plays a crucial role in the design of supercapacitors and electrode materials, whereas the understanding of the relevant mechanisms is still rather limited. Here, the quantum capacitance, work function and the related electronic and optical properties of eight functionalized MXenes Zr2CT2 monolayers (with functional group T = O, F, S, Cl, Br, OH, P and Se) are comprehensively investigated, by employing the first-principle calculations. All systems except Zr2CP2 are found to exhibit good stability. Modification with oxygen functional groups makes Zr2C change from metal to semiconductors. Surface functionalization modifies the work function of the MXenes by altering the Fermi level and the associated electron transfers. Particularly, the extremely low work function of Zr2C(OH)2 may enhance its power efficiency in optoelectronic fields. Introduction of functional groups may tune the density of electronic states near the Fermi level and effectively modulate the quantum capacitance and type of electrode materials. Zr2C suitable for symmetric supercapacitors and the modified systems applicable for potential anode or cathode materials are illustrated. The types of these electrode materials would not change in a wide voltage range. Further analyses on the optical properties and Bader charge are conducted for these materials.