An X-Ray Photoelectron Spectroscopy Study of Multilayered Transition Metal Carbides (MXenes)
Authors:   , Halim Joseph , Barsoum Michel W. 
Published: July 2021

Recently, a new family of two-dimensional (2D) early transition metal carbides and carbonitrides, labeled MXenes, was discovered at Drexel University. MXenes are produced by selectively etching mostly Al from Al-containing MAX phases and replacing Al with surface termination groups. Theoretically it has been predicted that changing the nature of the surface terminations, T, can change MXenes' properties, such as electronic and optical, resulting in changes in performance in various applications such as electrodes for Li-ion batteries. Prior to this work, there had been little systematic effort devoted to carefully identifying and quantifying the exact nature of T. In this work, high resolution XPS spectra of multilayered Ti₃C₂T_x, Ti₂CT_x, Ti₃CNT_x, Nb₂CT_x, Nb₄C₃T_x, V₂CT_x, Mo₂CT_x, Mo₂TiC₂T_x, and Mo₂Ti₂C₂T_x were acquired and analyzed. The influence of the M and X elements, the order of MXene, n, aging, Ar⁺ sputtering, and the concentration/nature of the etchants on the amounts of -O, -OH and -F, were systematically investigated. XPS analysis confirmed the intercalation of Li⁺ ions upon etching Ti₃AlC₂ using a mixture of LiF and HCl or LiCl and HF; when Ti₃C₂T_x multilayers is immersed in RbCl or NaCl solutions, cation exchange resulted in the replacement of Li⁺ ions with Rb⁺ or Na⁺, respectively. Similar cation intercalations were confirmed when treating HF-etched Ti₃C₂T_x samples with Ti₃C₂T_x with alkali metal hydroxides, such as NaOH and KOH. Based on quantification of the various chemical species before and after treatment, a mechanism was suggested where the reduction of -F terminations took place by exchanging with -OH and -O terminations and/or complete removal of Ti-F species and converting them to TiO₂-F_x. From XPS peak fits for all MXene samples in this study, it was established that x was ~ 2.0±0.2. Both -F and -OH terminations are formed during etching through the reaction of MXene with HF or H₂O, respectively. However, for the -O terminations to form, a mechanism was suggested were some of the -OH terminations dissociate into -O termination and H⁺, where the latter reacts with an intercalated H₂O forming [H₃O]⁺. The latter are then replaced with cations via ion exchange. This reaction has been suggested based on the trend that, for most MXenes analyzed, the ratio between -O terminations and adsorbed water plus cations is equal to unity or lower, suggesting that all H⁺ that dissociates from the -O terminations reacts with H₂O molecules to form H₃O⁺. Only five compounds out of the sixteen studied were found not to follow this trend (in those cases the ratio was higher than unity) where further investigation is needed to determine the cause of this anomaly. Given that recent work reported that the average oxidation state of Ti in Ti₃C₂T_x is + 2.4, the average oxidation state of C - for 8 different samples - was determined to be ~ -2.6±0.1. It is thus reasonable to conclude that the average oxidation states of Ti and C in Ti₃C₂T_x are weakly dependent on the etching conditions and/or intercalated cations. Similarly, the average oxidation state of C - for 2 different samples - was determined to be ~ -3.96±0.2 assuming the oxidation state of V determined in recent work to be +3.0. Based on this work, it is now possible to quantify the nature of the surface terminations in MXenes; information that can, in turn, be used to better design and tailor these novel 2D materials for various applications.