We have investigated the first stages of oxidation of the basal plane of MoS2 with O2. The different intermediates in the energy landscape were found at 0 K with nudged-elastic band calculations, and their reactivity was evaluated at higher temperatures by performing ab initio molecular dynamics simulations. We identified the intermediates and mechanisms leading to the desorption of both SO and SO2 species. The key intermediate consists of an O atom bound on top of an S atom with a second O atom inserted into the S–Mo bond, giving rise to a stable OS–O–Mo moiety. The mechanisms leading to this intermediate upon adsorption of O2 on the basal plane of MoS2 are discussed. From the OS–O–Mo intermediate, SO2 may desorb directly generating a single sulfur vacancy on the surface while its decomposition leads to the desorption of SO and leaves substitutional oxygen on the surface. These etching mechanisms were also observed in the ab initio molecular dynamics simulations, in good agreement with energy profiles calculated along the reaction paths. Diffusion of O atoms on top of the sulfur layer and direct desorption of SO groups were never observed in the molecular dynamics simulations because these processes have high energy barriers (2.4 and 3.1 eV, respectively). However, subsurface diffusion of O atoms, involving the formation of both O–S and O–Mo bonds, is a competing process with lower energy barriers.

The coalescence process of two nanoparticles to yield a core-shell structure is analyzed by a well-tempered metadynamics procedure. This methodology has been shown to be useful in understanding the present phenomenon in terms of two collective variables: the distance between the center of mass of the coalescing particles and the gyration radius of the resulting core element. The free-energy contour plots clearly show that the coalescence process involves the deformation of the core material, which is manifested in the residence of the system in regions with a larger gyration radius. Results from molecular dynamics for the same system were found helpful to reach the definition of this second collective variable. The advantages and limitations of the latter approach are discussed.

The reactions of O2 with S vacancy sites within a MoS2 monolayer were investigated using Density Functional Theory calculations. We considered the following defects: single S vacancy, double S vacancy,...