Title
Stability of 2H- and 1T-MoS2 in the presence of aqueous oxidants and its protection by a carbon shell
Author
Randal Marks, Andrew Schranck, Roy Stillwell and Kyle Doudrick
Year
2020
Journal
RSC Advances
Abstract
Two-dimensional molybdenum disulfide (MoS2) is emerging as a catalyst for energy and environmental applications. Recent studies have suggested the stability of MoS2 is questionable when exposed to oxidizing conditions found in water and air. In this study, the aqueous stability of 2H- and 1T-MoS2 and 2H-MoS2 protected with a carbon shell was evaluated in the presence of model oxidants (O2, NO2 , BrO3 ). The MoS2 electrocatalytic performance and stability was characterized using linear sweep voltammetry and chronoamperometry. In the presence of dissolved oxygen (DO) only, 2H- and 1T-MoS2 were relatively stable, with SO4 2 formation of only 2.5% and 3.1%, respectively. The presence of NO2 resulted in drastically different results, with SO4 2 formations of 11% and 14% for 2H- and 1T-MoS2, respectively. When NO2 was present without DO, the 2H- and 1T-MoS2 remained relatively stable with SO4 2 formations of only 4.2% and 3.3%, respectively. Similar results were observed when BrO3 was used as an oxidant. Collectively, these results indicate that the oxidation of 2H- and 1T-MoS2 can be severe in the presence of these aqueous oxidants but that DO is also required. To investigate the ability of a capping agent to protect the MoS2 from oxidation, a carbon shell was added to 2H–MoS2. In a batch suspension in the presence of DO and NO2 , the 2H–MoS2 with the carbon shell exhibited good stability with no oxidation observed. The activity of 2H–MoS2 electrodes was then evaluated for the hydrogen evolution reaction by a Tafel analysis. The carbon shell improved the activity of 2H–MoS2 with a decrease in the Tafel slope from 451 to 371 mV dec 1 . The electrode stability, characterized by chronopotentiometry, was also enhanced for the 2H–MoS2 coated with a carbon shell, with no marked degradation in current density observed over the reaction period. Because of the instability exhibited by unprotected MoS2, it will only be a useful catalyst if measures are taken to protect the surface from oxidation. Further, given the propensity of MoS2 to undergo oxidation in aqueous solutions, caution should be used when describing it as a true catalyst for reduction reactions (e.g., H2 evolution), unless proven otherwise
Instrument
NRS5100
Keywords
Raman imaging microscopy, aqueous oxidants, carbon shell, 1T-MoS2