MXenes are a new, and growing, family of 2D materials with very promising properties for a wide variety of applications. Obtained from the etching of MAX phases, numerous properties can be targeted thanks to the chemical richness of the precursors. Herein, we highlight how etching agents govern surface chemistries of Ti 3 C 2 T x , the most widely studied MXene to date. By combining characterization tools such as X-ray diffraction, X-ray photoelectron, Raman and electron energy loss spectroscopies, scanning and transmission electron microscopies and a surface sensitive electrochemical reactionthe hydrogen evolution reaction, HERwe clearly demonstrate that the etching agent (HF, LiF/HCl or FeF 3 /HCl) strongly modifies the nature of surface terminal groups (F, OH and/or O), oxidation sensitivity, delamination ability, nature of the inserted species, interstratification, concentration of defects and size of flakes. Beyond showing how using these different characterization tools to analyze MXenes, this work highlights that the MXene synthesis routes can influence targeted applications.

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MXenes are a young family of two-dimensional transition metals carbides, nitrides, and carbonitrides with highly controllable structure, composition, and surface chemistry to adjust for target applications. Here, we demonstrate the modifications of two-dimensional MXenes by low-energy ion implantation, leading to the incorporation of Mn ions in Ti3C2Tx thin films.Damages and structural defects caused by the implantation process are characterized at different depths by XPS on Ti2p core-level spectra, ToF-SIMS and with electron energy loss spectroscopy analyses. Results show that the ion-induced alteration of the damage tolerant Ti3C2Tx layer is due to defects formation in both Ti and C sites, thereby promoting the functionalization of these sites with oxygen groups. This work contributes to the inspiring approach of tailoring 2D MXenes structure and properties through doping and defects formation by low-energy ion implantation to expand their practical applications.

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MXenes are single or few-layered two-dimensional (2D) transition metals carbides and/or nitrides successfully synthesized in 2011, with chemical composition Mn+1XnTx, where M stands for an early transition metal, X is C and/or N, Tx are a surface terminations and n = 1, 2, or 3 [1]. These materials are considered as promising candidates in many applications such as energy storage, sensing, catalysis, and optoelectronics. For instance, MXenes combine high conductivity and hydrophilicity making them interesting candidates for applications involving aqueous media and requiring good electronic properties or easy processing. Moreover, they exhibit a wide chemical variability thanks to their parent MAX phases precursors, a 150-plus member family of layered ternary carbides or nitrides with chemical composition Mn+1AXn (where the A-element mainly belong to the columns 13 and 14 of the periodic table). This offers the unique opportunity to design the MXenes on demand, with considerably different physicochemical properties for target applications [2-4]. Tuning the MXene properties is also possible through the manipulation of their surface termination groups T (i.e., T = −F, −OH, −O, or −Cl). The crucial role of the surface groups has recently triggered many studies aiming at controlling and characterizing the MXene surface structure/chemistry mainly by using different etching protocols [5]. However, the nature of the surface groups, inherited from the A element exfoliation step of the MAX phases by chemical reactions, is hardly tunable by this mean except by changing the etching protocole which might have significant impact on the quality of the synthetized MXenes. Here, we demonstrate that medium energy ion implantation can be used to modify the MXene surface functionalization, through the controlled formation of structural defects and the incorporation of foreign species in Ti3C2Tx thin films fabricated by spin coating (Fig.1 a-b). Defects created by Mn implantation in a wide fluence range (1x1014-5x1016 cm-2) are characterized at different depths by XPS on Ti2p core-level spectra, ToF-SIMS and EELS analyses. For the latest, we use an original approach that we have developed [6], allowing an estimation of the level of damage in the material by studying the C-K edge. Our results show that the 60 keV Mn implantation leads to successful incorporation of Mn in the Ti3C2 thin film. This is accompanied by the structural distortion of the Ti-C layers as evidenced by EELS experiments (Fig. 1c), preferential Ti sputtering as compared to C without any significant amorphization or phase transformation up to a fluence of 1x1016 cm-2. However, for the highest fluence studied (5x1016 cm-2), complete amorphization was observed. For the subthreshold amorphization fluence, the study of the fine structure of the O K-edge and of the electron diffraction pattern suggest the formation of extended defects in the Ti layers including the occupation of the defective Ti sites by oxygen, leading to local environment for the oxygen atoms like graphene oxide. Finally, our results show that the number of defects can be directly and in a reproductible way tunned by the implantation fluence. [1] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M.W. Barsoum, Adv. Mater. 2011, 23, 4248−4253 [2] Y. Gogotsi, B. Anasori, ACS Nano 2019, 13, 8491−8494 [3] Q. Tao, M. Dahlqvist, J. Lu, S. Kota, R. Meshkian, J. Halim, J. Palisaitis, L. Hultman, M. W. Barsoum, P. O. Persson, J. Rosen, Nat. Commun. 2017, 8, No. 14949. [4] B. Anasori, C. Shi, E.J. Moon, Y. Xie, C.A. Voigt, P. R. Kent, S.J. May, S.J. Billinge, M.W. Barsoum, Y. Gogotsi, Nanoscale Horiz. 2016, 1, 227−234 [5] M. Benchakar, L. Loupias, C. Garnero, T. Bilyk, C. Morais, C. Canaff, N. Guignard, S. Morisset, H. Pazniak, S. Hurand, et al. Appl. Surf. Sci. 2020, 530, No. 147209 [6] T. Bilyk, M. Benchakar, M. Bugnet, L. Loupias, P. Chartier, H. Pazniak, M.-L. David, A. Habrioux, S. Celerier, J. Pacaud, and V. Mauchamp, J. Phys. Chem. C (to be published, https://dx.doi.org/10.1021/acs.jpcc.0c06798) Figure 1

Like many 2D materials, numerous properties of MXene multilayers, and especially the most popular one Ti3C2T x , have been shown to significantly depend on their architecture, i.e. the number of layers and interlayer distance. These structural parameters are thus key elements to be characterized for the analysis of MXene properties. Focusing on valence electron energy-loss spectroscopy (VEELS) as performed in a transmission electron microscope (TEM), and using density functional theory (DFT) simulations, we here analyze the layer dependent large changes in the VEEL spectra of Ti3C2T x multilayers as a probe of their total thickness, and emphasize the bulk plasmon energy sensitivity to interlayer distance. Together these findings allow to directly quantify the absolute number of layers in a Ti3C2T x stack up to ∼10 nm thickness and give access to interlayer distance modifications with sub-angström sensitivity, evidencing VEELS as a powerful method for the characterization of MXene multilayers on the nanometer scale. We expect these results to be relevant for the study of structure/properties correlations in this class of materials, especially with the development of in situ or environmental TEM experiments.