Local chemical bonding and structural properties in Ti3Al C2 MAX phase and Ti3C2Tx MXene probed by Ti 1 s x-ray absorption spectroscopy
The chemical bonding within the transition-metal carbide materials MAX phase Ti3Al C2 and MXene Ti3C2Tx is investigated by x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure (EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers of Mn +1Xn and monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where M is a transition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination species Tx will separate the Mn +1Xn -layers forming two-dimensional (2D) flakes of Mn +1XnTx . For Ti3C2Tx the Tx corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D Mn +1Xn -flake. The Ti K-edge (1 s ) XANES of both Ti3Al C2 and Ti3C2Tx exhibit characteristic preedge absorption regions of C 2 p -Ti 3 d hybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti 1 s →4 p excitation, where only the latter shows sensitivity toward the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti3Al C2 and Ti3C2Tx are highly anisotropic with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750 °C.