X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes)

In this work, a detailed high resolution X-ray photoelectron spectroscopy (XPS) analysis is presented for select MXenes-a recently discovered family of two-dimensional (2D) carbides and carbonitrides. Given their 2D nature, understanding their surface chemistry is paramount. Herein we identify and quantify the surface groups present before, and after, sputter-cleaning as well as freshly prepared vs. aged multi-layered cold pressed discs. The nominal compositions of the MXenes studied here are Ti₃C₂T_x, Ti₂CT_x, Ti₃CNT_x, Nb₂CT_x and Nb₄C₃T_x, where T represents surface groups that this work attempts to quantify. In all the cases, the presence of three surface terminations, sbnd O, sbnd OH and sbnd F, in addition to OH-terminations relatively strongly bonded to H₂O molecules, was confirmed. From XPS peak fits, it was possible to establish the average sum of the negative charges of the terminations for the aforementioned MXenes. Based on this work, it is now possible to quantify the nature of the surface terminations. This information can, in turn, be used to better design and tailor these novel 2D materials for various applications.