Gas Sensing and Half-Metallic Materials Design Using Metal Embedded into S Vacancies in WS(2) Monolayers: Adsorption of NO, CO, and O(2) Molecules

The adsorption of CO, NO, and O(2) molecules onto Cu, Ag, and Au atoms placed in the S vacancies of a WS(2) monolayer was elucidated within dispersion-corrected density functional theory. The binding energies computed for embedded defects into S vacancies were 2.99 (Au(S)), 2.44 (Ag(S)), 3.32 eV (Cu(S)), 3.23 (Au(2S2)), 2.55 (Ag(2S2)), and 3.48 eV/atom (Cu(2S2)), respectively. The calculated diffusion energy barriers from an S vacancy to a nearby site for Cu, Ag, and Au were 2.29, 2.18, and 2.16 eV, respectively. Thus, the substitutional atoms remained firmly fixed at temperatures above 700 K. Similarly, the adsorption energies showed that nitric oxide and carbon oxide molecules exhibited stronger chemisorption than O(2) molecules on any of the metal atoms (Au, Cu, or Ag) placed in the S vacancies of the WS(2) monolayer. Therefore, the adsorption of O(2) did not compete with NO or CO adsorption and did not displace them. The density of states showed that a WS(2) monolayer modified with a Cu, Au, or Ag atom could be used to design sensing devices, based on electronic or magnetic properties, for atmospheric pollutants. More interestingly, the adsorption of CO changed only the electronic properties of the MoS(2)-Au(S) monolayer, which could be used for sensing applications. In contrast, the O(2) molecule was chemisorbed more strongly than CO or NO on Au(2S2), Cu(2S2), or Ag(2S2) placed into di-S vacancies. Thus, if the experimental system is exposed to air, the low quantities of O(2) molecules present should result in the oxidation of the metallic atoms. Furthermore, the O(2) molecules adsorbed on WS(2)-Au(2S2) and WS(2)-Cu(S) introduced a half-metallic behavior, making the system suitable for applications in spintronics.