MXenes (pronounced “maxenes”) are 2D materials that can host a rich array of distinct chemical compositions that can be tuned for a broad range of applications, from energy storage to water purification. Described by the general chemical formula, Mn+1XnTx (where M = transition metal, X = carbon or nitrogen, and T = surface functional groups), these compounds are commonly synthesized from a parent Mn+1AlXn (MAX) phase. The surface groups are known to directly affect MXene functional properties; however, a detailed understanding of how the surface groups influence electronic structure within distinct MXene layers has been lacking.
Using x-ray absorption spectroscopy at ALS Beamline 4.0.2 and NSLS-II Beamline 23-ID-2, researchers studied MXenes in which the transition metal Ti is located at surface sites (Ti2CTx and Ti3C2Tx) and those with Ti at subsurface sites only (“double-M” MXenes: Mo2TiC2Tx and Cr2TiC2Tx). The impact of the surface species on electronic structure was measured by comparing the Ti L-edge spectra of both types with that of their parent MAX compounds (i.e., before addition of the surface groups). The results, confirmed by spectral analysis and theory-based calculations, showed that the surface groups alter the electronic structure of Ti at the surface, but not below it.
Based on these observations, the researchers proposed that double-M MXenes offer a pathway toward engineering expanded functionality that’s independent of surface groups, by having the subsurface M-site layers host the desired electronic, magnetic, or topological properties. In the future, the researchers plan to explore this design strategy to realize MXenes with robust magnetic ordering.
![Spectra for MXene with surface titanium show large deviations after the addition of surface groups.](https://als.lbl.gov/wp-content/uploads/2020/09/2020-09-30-yang.jpg)
Y. Yang, K. Hantanasirisakul, N. Frey, B. Anasori, R.W. Green, P. Rogge, I. Waluyo, A. Hunt, P. Shafer, E. Arenholz, V. Shenoy, Y. Gogotsi, and S.J. May, “Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes,” 2D Mater. 7, 025015 (2020), doi:10.1088/2053-1583/ab68e7. The experimental work was supported by the DOE Office of Science, grant no. DE-SC0018618.