First-principles study of electronic, optical properties, workfunction and quantum capacitance of Janus Hf2COT MXene

Janus MXene is attracting more and more interest because of its special characteristics. The electronic and optical properties, work function and quantum capacitance of Janus Hf2COT (T = -Br, -Cl, –F, –OH, -P, –S, -Se) MXene are investigated using density functional theory. The analysis of binding energies and formation energies indicate their structural stability. The asymmetric functional group changes Hf2CO2 from a semiconductor to metal Hf2COT. Hf2COF and Hf2COOH have larger absorption coefficient and conductivities in infrared region, which have potential applications for optoelectronic devices. The Janus surface groups can effectively increase the quantum capacitance and further change the electrode type of the system. In aqueous system, Hf2CO2, Hf2COBr, Hf2COCl and Hf2COP are preferred for cathode electrode material, Hf2COOH, Hf2COS and Hf2COSe are preferred for anode material. The quantum capacitance in ionic/organic system is further explored. The electronic properties and quantum capacitance of Hf2C with mixed terminations are also investigated.