Pit Formation Mechanism of Monolayer MoS₂ By Thermal Oxidation

Abstract

Generating pits and thereby active edge sites of molybdenum disulfide (MoS2) monolayers is desirable for many electrochemical catalytic reactions including hydrogen evolution reaction (HER). Thermal oxidation is one of the potentially scalable and facile methods to effectively create the pits on MoS2 monolayers. Therefore understanding the thermal oxidation mechanism is very important to precisely control the generation of active edge sites of MoS2-based electrocatalysts. To date, pits are assumed to be favorably formed on MoS2 at undercoordinated sites such as sulfur (S) vacancies at high temperatures. However, the thermal oxidation studies have not considered the existence of adventitious carbon (C) that exists almost everywhere and interact with oxygen at elevated temperatures. Herein, we investigated the influence of adventitious C on the thermal oxidation of MoS2 monolayers. We employed in situ environmental transmission electron microscopy (ETEM) to demonstrate the pit formation mechanism with the presence of adventitious C at the oxidation temperature of 300 °C. The in situ ETEM results show that the adventitious C is agglomerated at high temperatures. Then, the interfaces between monolayer MoS2 and C nanoparticle provide preferred sites for thermal oxidation of MoS2 and thus pit formation while the individual S vacancies exist intact. Density functional theory (DFT) calculations show the interfaces between MoS2 and C nanoparticle make the sequential adsorption of oxygen atoms thermodynamically favorable, unlike only S vacancy sites. We also tested the electrochemical performance of MoS2 with pits for HER and it reduced the overpotential by ~ 130 mV. These results demonstrate the combination between ETEM experiment and DFT calculation can be effectively used to study the chemical reaction mechanism between 2-dimensional materials such as MoS2 and gaseous species. Furthermore, we provide the potential to control the active edge site of MoS2-based electrocatalyst by understanding the fundamental pit formation mechanism by thermal oxidation.