https://doi.org/10.1016/j.xcrp.2022.101151
“The titanium carbide Ti3C2Tx (Tx = surface functional groups) MXene exhibits promising physical properties and chemical activity as one of the most commonly and extensively investigated materials, but the participating surface structures and properties have yet to be clarified. Here, we reveal the superficial nature of etching-processed Ti3C2Tx using transmission electron microscopy/spectroscopy, first-principles calculation, and molecular dynamics simulation. An O-rich, but C-deficient, diffusion layer (approximately 3 nm) with an inverse gradient distribution is identified. Associated with the superficial substitution of O for C, an amorphous overlayer (approximately 2 nm) containing oxyfluoride of Ti, Al, and C is also revealed. The stable diffusion layer has a high density of states at the Fermi level and a low shear modulus to bulk modulus ratio, implying that the formation of the diffusion layer is not detrimental to Ti3C2Tx’s conductivity and ductility. These findings are significant for improving oxidation strategies and understanding the structural properties of this material.”
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Ti3C2Tx and its potential oxidation
Selective etching by Al-layer extraction from Ti3AlC2 was performed using an HF·H2O solution to fabricate the target Ti3C2Tx, as shown schematically in Figure 1A. Therefore, the resulting Ti3C2Tx has an accordion-shaped structure with surface Ti3C2 layers bonded to numerous functional groups (Tx), such as O, OH, and F. Ionic tunnels formed because of the expanded interlayer spacing between two neighboring Ti3C2Tx (Figure 1B), allowing Ti3C2Tx to fully exploit the available functions, such as electrochemical redox activity for electroactive Ti cations on both sides of the [105]-oriented TiC6 octahedron.24
Figure 1. Fabrication, microstructure, and potential oxidation of Ti3C2Tx
(A) Etching with HF·H2O governs the fabrication procedures.
(B) Ionic-tunnel-structure characteristic.
(C–J) [100]-oriented ADF images demonstrate cross-sectional microstructural changes as a function of etching durations: (C and F) unetched Ti3AlC2; (D and G) 4-day etching; (E and H) 17-day etching. The variations in ionic tunnel width measured with line profiles extracted from the corresponding dashed rectangle areas of the samples are shown in the insets in (C, D, and E). MD simulations of Ti3C2Tx (001) plane oxidation states with achieving O2 at 1,100 K for (I) initial state and (J) 20 ps. After oxidation, the outmost layer of Ti3C2(OH)2 exhibits superficial disordering (indicated by a) and reduced crystal thickness (indicated by c).
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