Local chemical bonding and structural properties in Ti3AlC2 MAX phase and Ti3C2Tx MXene probed by Ti 1s X-ray absorption spectroscopy

TL;DR

This study uses X-ray absorption spectroscopy to analyze the chemical bonding and structural properties of Ti3AlC2 MAX phase and Ti3C2Tx MXene, revealing detailed insights into their local atomic environments and effects of temperature.

Contribution

It provides a detailed spectroscopic comparison of MAX phase and MXene, highlighting how surface terminations influence their local structure and bonding.

Findings

Ti K-edge XANES shows characteristic hybridization and crystal-field splitting.

EXAFS reveals highly anisotropic local structures with in-plane and out-of-plane contributions.

Temperature affects bond lengths and termination species coverage.

Abstract

The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2 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 (1s) XANES of both Ti3AlC2 and Ti3C2Tx exhibit characteristic pre-edge absorption regions of C 2p - Ti 3d hybridization with clear crystal-field splitting's while the main-edge absorption features originate from the Ti 1s -> 4p excitation, where only the latter shows sensitivity towards the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti3AlC2 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 {\deg}C.