A critical analysis of the X-ray photoelectron spectra of Ti3C2Tz MXenes

• Published in: Matter Vol. 4; no. 4; pp. 1224 - 1251
• Main Authors: Natu, Varun, Benchakar, Mohamed, Canaff, Christine, Habrioux, Aurélien, Célérier, Stéphane, Barsoum, Michel W.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 07.04.2021; Cell Press
• Subjects: Chemical Sciences; MXene; Physics; Ti3C2; XPS; 2D; XPS; Ti3C2; MXene
• ISSN: 2590-2385; 2590-2385
• DOI: 10.1016/j.matt.2021.01.015

Since their discovery in 2011, MXenes have garnered worldwide interest. Given their 2D structure, surface, or termination, chemistries play a vital role in most applications. X-ray photoelectron spectroscopy (XPS) is one of the most common characterization tools for quantifying surface terminations and overall chemistry. Herein, we critically review the XPS fitting models proposed for Ti3C2Tz MXene in the literature and make the case that they are at best incomplete and at worst contradictory. We propose a new fitting algorithm based on all the data obtained from previously published studies and propose a new method for quantifying the surface terminations in Ti3C2Tz. In our approach, we assign the Ti 2p peak at 455.1 eV to the C–Ti–O\O\O, and the peaks at 456.0, 457.0, 457.9, and 459.6 eV are assigned to C–Ti–O\O\F, C–Ti–O\F\F, C–Ti–F\F\F, and TiO2-xF2x, respectively. The first four represent possible Ti atom terminations; the last is an oxyfluoride. The C 1s peak at 282 eV, ascribed to C atoms surrounded by 6 Ti atoms, is so universal that it can almost be used as a reference. [Display omitted] Interest in MXenes is quite high because of their potential in a wide range of applications. Although several MXenes have been discovered to date, most studies so far have focused mainly on Ti3C2Tz. Given their 2D nature and rich surface chemistry, XPS is the preferred technique for analyzing their compositions. To date, only four studies have focused on understanding and fitting the XPS spectra on Ti3C2Tx flakes. While there are commonalities between the various fitting protocols, there are also some inconsistencies that have sown confusion. In this review, we summarize the XPS fitting models proposed especially for Ti3C2Tz and propose a new fitting model based on critical analysis of previously published studies. We also point out areas where more work is needed to better understand the XPS spectra. We feel this article will help researchers to better analyze their data using XPS for MXenes and other 2D materials. Since their discovery in 2011, MXenes have garnered a lot worldwide interest. Given their 2D structure, surface terminations/chemistries play a vital role in most applications. X-ray photoelectron spectroscopy (XPS) is one of the most common characterization tools used for quantifying these surface terminations and overall chemistry. In this work, we critically review the XPS fitting models proposed for Ti3C2Tz MXene in the literature and propose a new fitting algorithm based on all the data obtained from previously published studies.