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.
Highlights
Despite the large interest in graphene [1], which lacks a natural band gap, it has been difficult to artificially produce graphene-based materials with suitable band gaps
MXenes consist of a core of a few atoms thick 2D Mn+1Xn conductive carbide layer that is crystalline in the basal plane and a transition-metal surface that can be functionalized for different material properties by changing the chemistry of the termination species
The additional main-edge energy shift caused by the heat treatment, cannot be a consequence of a further withdrawal of charge from the Ti atoms, because that would contradict the previous temperature-programmed x-ray photoelectron spectroscopy (XPS) study [18]; the intensity at the high-binding-energy side of the Ti 2p3/2 XPS spectra decreases while F desorbs, indicating that Ti in Ti3C2Tx chemically reduces in a heat treatment
Summary
Despite the large interest in graphene [1], which lacks a natural band gap, it has been difficult to artificially produce graphene-based materials with suitable band gaps. MXenes consist of a core of a few atoms thick 2D Mn+1Xn conductive carbide layer that is crystalline in the basal plane and a transition-metal surface that can be functionalized for different material properties by changing the chemistry of the termination species. Despite vast interest in MXenes in general, and in Ti3C2Tx in particular, there is little known experimentally about the bonding between the transition metals and the terminating species, Tx. Previous work of MXenes’ electronic structure has mainly been based on ground-state density functional theory (DFT) calculations at 0 K [14,15,16,17,19]. The change of the fcc coordination will induce modifications in the XANES and EXAFS spectra, it will reveal local information on the bonding situation between the transition-metal atoms and termination species. The study demonstrates how x-ray absorption spectroscopy can be used to probe the MXene surfaces to shed more light on the local chemical interaction between F, O, and Ti atoms, which is relevant knowledge when designing MXenes for sought-after material properties
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