2, The Pennsylvania State University, University Park, Pennsylvania, United States
3, Drexel University, Philadelphia, Pennsylvania, United States
MXenes are two-dimensional transition metal carbides or nitrides that have recently gained interest with applications geared towards energy storage, catalysis, and electronic devices. However, until now, bottom-up synthesis of MXenes have not been reported. In this work, using in situ aberration-corrected scanning transmission electron microscopy (STEM), we observed homoepitaxial growth of an additional hexagonal TiC (h-TiC) layer on both surfaces of 2D MXene (Ti3C2) monolayer flakes, forming new 2D MXenes Ti4C3 and Ti5C4, at temperature above 500 °C. The growth of single-layer h-TiC is controlled by a small diffusion barrier and a large step-edge barrier as revealed by density functional theory (DFT) and ReaxFF molecular dynamics simulations. The in situ heating experiments also reveal the edge structure of the MXene and the h-TiC add-layer, and the unique properties of the edges are understood using DFT. These findings thus provide insights on MXenes growth and pave the way to fabricate MXenes with controlled morphology for tailored functionality.
Research supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Aberration-corrected STEM imaging was conducted as part of a user proposal at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy Office of Science User Facility.