Stereochemical Vacuum Gap Explains Out-of-Plane Thermal Insulation in MXenes
Two-dimensional MXenes are promising materials for thermal management and spectral camouflage, combining low out-of-plane thermal conductivity with low infrared emissivity and mechanical robustness. Yet the near-order-of-magnitude spread in experimental out-of-plane thermal conductivity measurements (0.14-0.8 W/mK) and the systematic overestimation by simulations point to a fundamental gap in our understanding of heat transport in these materials. Here, we argue these differences originate in the overlooked role of heterogeneous surface terminations. Using Non-Equilibrium Molecular Dynamics simulations of Ti3C2Tx, we show that this discrepancy arises from a stereochemically induced vacuum gap between adjacent layers, formed when surface terminations of different sizes coexist. Even minor deviations from homogeneous terminations drastically suppress out-of-plane thermal conductivity, bringing simulated values into quantitative agreement with experiment. We also show that thermal conductivity scales strongly with the atomic density, and that introducing bulky surface terminations, including residual water, reduces the thermal conductivity to 0.3 W/mK, an order of magnitude below homogeneous termination values and below the minimum thermal conductivity limit predicted for disordered solids. Thus, we propose a chemistry-driven route to engineer thermal transport in MXenes.