Phase behavior of terminating oxygen layer of V2C MXene.

The investigation is made on the phase behavior of the surface-terminating oxygen layer on MXene V2C sheet using the methods of density functional theory (DF)T and statistical physics. Two approximations of the potential of lateral interactions between adsorbed oxygen atoms are constructed. The first approximation is the pair potential form. The second approximation is constructed by the Cluster Expansion (CE) method. As expected, quantitative estimations of approximation errors show that the CE approximation is more accurate, but the pair potential also turns to be of comparable accuracy. Then, adsorption isotherms are calculated using statistical physics algorithms implemented in the SuSMoST code. The isotherms reveal nontrivial phase behavior of the oxygen layer: three phases with coverages of ⅓, ½ and ⅔ monolayer are discovered. From free energy calculations, it is predicted that complete cleaning of the V2C surface from oxygen is possible at temperatures above 2500 K and at partial pressure below 10−9 bar. At atmospheric pressure in air, a complete oxygen coating is stable at temperatures up to 2700 K. Phases with intermediate coverages are stable at temperature below 1200 K and probably could be observed only at ultra-low partial pressure of oxygen. [ABSTRACT FROM AUTHOR]