Local chemical bonding and structural properties in Ti3AlC2 MAX phase and Ti3C2Tx MXene probed by Ti 1s x-ray absorption spectroscopy
The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2 and MXene Ti3C2Txis 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 layersof Mn+1Xn and monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where M is atransition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination speciesTx 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(1s) XANES of both Ti3AlC2 and Ti3C2Tx exhibit characteristic preedge absorption regions of C 2p-Ti 3dhybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti1s → 4p excitation, where only the latter shows sensitivity toward the fcc-site occupation of the terminationspecies. The coordination numbers obtained from EXAFS show that Ti3AlC2 and Ti3C2Tx are highly anisotropicwith a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayersand termination species, respectively. As shown in the temperature-dependent measurements, the O contributionshifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750 °C. ; Funding agencies: Swedish Research CouncilSwedish Research Council [2018-07152]; Swedish Governmental Agency for Innovation SystemsVinnova [2018-04969]; FormasSwedish Research Council Formas [2019-02496]; Swedish Research Council (VR) LiLi-NFM Linnaeus EnvironmentSwedish R