The influence of surface functionalization on thermal transport and thermoelectric properties of MXene monolayers

Abstract

The newest members of a two-dimensional material family, involving transition metal carbides and nitrides (called MXenes), have garnered increasing attention due to their tunable electronic and thermal properties depending on the chemical composition and functionalization. This flexibility can be exploited to fabricate efficient electrochemical energy storage (batteries) and energy conversion (thermoelectric) devices. In this study, we calculated the Seebeck coefficients and lattice thermal conductivity values of oxygen terminated M₂CO₂ (where M = Ti, Zr, Hf, Sc) monolayer MXene crystals in two different functionalization configurations (model-II (MD-II) and model-III (MD-III)), using density functional theory and Boltzmann transport theory. We estimated the thermoelectric figure-of-merit, zT, of these materials by two different approaches, as well. First of all, we found that the structural model (i.e. adsorption site of oxygen atom on the surface of MXene) has a paramount impact on the electronic and thermoelectric properties of MXene crystals, which can be exploited to engineer the thermoelectric properties of these materials. The lattice thermal conductivity κ_l, Seebeck coefficient and zT values may vary by 40% depending on the structural model. The MD-III configuration always has the larger band gap, Seebeck coefficient and zT, and smaller κ_l as compared to the MD-II structure due to a larger band gap, highly flat valence band and reduced crystal symmetry in the former. The MD-III configuration of Ti₂CO₂ and Zr₂CO₂ has the lowest κ_l as compared to the same configuration of Hf₂CO₂ and Sc₂CO₂. Among all the considered structures, the MD-II configuration of Hf₂CO₂ has the highest κ_l, and Ti₂CO₂ and Zr₂CO₂ in the MD-III configuration have the lowest κ_l. For instance, while the band gap of the MD-II configuration of Ti₂CO₂ is 0.26 eV, it becomes 0.69 eV in MD-III. The zT_max value may reach up to 1.1 depending on the structural model of MXene.