XPS Peak-Fitting of 2H MoS <sub>2</sub> , 1T MoS <sub>2</sub> , and MoS <sub>2-X</sub> Nanosheets in MoS <sub>2</sub> Powders and Battery Electrodes After Ar <sup>+</sup> Ion Depth-Profiling
Alexandar D. Marinov, Adam J. Clancy, Christopher A. Howard, Patrick L. Cullen
Abstract
High Resolution Image Download MS PowerPoint Slide The two common characterization techniques that can distinguish between the metastable 1T and stable 2H phase of molybdenum disulfide (MoS 2 ) are Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Argon ion etching within XPS offers the possibility to explore sample composition as a function of depth. However, for 2H MoS 2 samples this results in sample alteration via the creation of sulfur vacancies and leads to local areas of MoS 2- x and 1T/2H MoS 2 . XPS MoS 2- x generation (228.1 eV) and laboratory nanoscale 1T MoS 2 synthesis (228.4 eV) are easily mistaken in XPS, as both signals exhibit downshifted Mo 3d binding energies relative to 2H MoS 2 (229.3 eV). Thus, we applied a four split orbit peak XPS model that enables distinction between MoS 2- x alteration and successful 1T phase synthesis. In argon etching induced MoS 2- x the pristine S 2p peaks remain dominant (162.0 eV) and the S/Mo atomic ratio decreases from 2.5 to 1.1 after electrode ion bombardment. When nanoscale 1T MoS 2 forms during lithiation in a lithium-ion battery, the dominant S 2p peaks are found at lower binding energies (161.5 eV) and the S/Mo atomic ratio is elevated (>2.0). Depth profiling of ex situ 1T/2H MoS 2 electrodes showcases lithiation beyond the electrode surface, clearly distinguished from solely MoS 2- x alteration through the S 2p component percentage, which exceeds the 1T MoS 2 limiting threshold (1T > 13% of S 2p) for ion bombarding of as-cast electrodes.