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.