Atomistic Detail of the Formation of WSO and WSeO Janus Monolayers and Their Role for Cation Selection: Toward Effective Materials for Environmental Remediation

Transition metal dichalcogenides (TMDs), in their 2D form, are a family of semiconductors with great versatility and several applications, spanning from environmental, energy, electronics, and even spintronics. This work focuses on WS₂ and WSe₂ monolayers (MLs) and their oxidation process to obtain WSO and WSeO using quantum mechanical calculations. The oxidation occurs in well-ordered thermodynamically viable diagonal patterns. There is also a lattice parameter reduction with a linear behavior when going from WS₂/WSe₂ toward WSO/WSeO; this fact is directly related to the electronegativity of the chalcogen species (Se > S > O). All oxidized structures exhibited bonds with mixed covalent and ionic characteristics, with the W-O bond displaying the stronger ionic character. We tested the capacity of nonoxidized and oxidized monolayers as agents for ion trapping. Pristine WS₂ and WSe₂ monolayers effectively adsorbed Ca, K, Na, and Cl, with the lowest adsorption observed for Mg. In contrast, the Janus monolayers exhibited apparent selectivity toward cations and showed the lowest adsorption energy for the Cl anion under gas-phase conditions. Under aqueous conditions, the same behavior was observed for Ca, Mg, K, and Na, which form chemical bonds with the oxidized substrate while still coordinating with their hydration shells. In contrast, Cl interacts weakly with the surface, as it prefers to interact with its hydration shell. Electron localization function analysis demonstrated the ionic bond formation in the cations, and the noncovalent interaction index isosurfaces clarified the weak van der Waals (vdW) interactions that hold the Cl-surface interaction, being a strong proof that the oxidized part of the Janus WSO and WSeO monolayers evidence cation selection, which points to these 2D materials as potential anodes for water treatment.