Stability of 2H- and 1T-MoS 2 in the presence of aqueous oxidants and its protection by a carbon shell.

Two-dimensional molybdenum disulfide (MoS 2 ) is emerging as a catalyst for energy and environmental applications. Recent studies have suggested the stability of MoS 2 is questionable when exposed to oxidizing conditions found in water and air. In this study, the aqueous stability of 2H- and 1T-MoS 2 and 2H-MoS 2 protected with a carbon shell was evaluated in the presence of model oxidants (O 2 , NO 2 - , BrO 3 - ). The MoS 2 electrocatalytic performance and stability was characterized using linear sweep voltammetry and chronoamperometry. In the presence of dissolved oxygen (DO) only, 2H- and 1T-MoS 2 were relatively stable, with SO 4 2- formation of only 2.5% and 3.1%, respectively. The presence of NO 2 - resulted in drastically different results, with SO 4 2- formations of 11% and 14% for 2H- and 1T-MoS 2 , respectively. When NO 2 - was present without DO, the 2H- and 1T-MoS 2 remained relatively stable with SO 4 2- formations of only 4.2% and 3.3%, respectively. Similar results were observed when BrO 3 - was used as an oxidant. Collectively, these results indicate that the oxidation of 2H- and 1T-MoS 2 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 MoS 2 from oxidation, a carbon shell was added to 2H-MoS 2 . In a batch suspension in the presence of DO and NO 2 - , the 2H-MoS 2 with the carbon shell exhibited good stability with no oxidation observed. The activity of 2H-MoS 2 electrodes was then evaluated for the hydrogen evolution reaction by a Tafel analysis. The carbon shell improved the activity of 2H-MoS 2 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-MoS 2 coated with a carbon shell, with no marked degradation in current density observed over the reaction period. Because of the instability exhibited by unprotected MoS 2 , it will only be a useful catalyst if measures are taken to protect the surface from oxidation. Further, given the propensity of MoS 2 to undergo oxidation in aqueous solutions, caution should be used when describing it as a true catalyst for reduction reactions ( e.g. , H 2 evolution), unless proven otherwise.