DFT exploration of the sensing performance of Janus transition metal dichalcogenides (ASSe; A = Sc, Ti, Zr, Nb, Pd) monolayers for HCN, H2S, and NH3 toxic gases.

Numerous toxic gases such as hydrogen cyanide (HCN), hydrogen sulfide (H2S) and ammonia (NH3) pose severe threats to environmental safety and human health, necessitating the development of highly sensitive and selective gas sensors. This study evaluates the adsorption behavior and sensing capabilities of five Janus transition metal dichalcogenide (TMD) monolayers (ScSSe, TiSSe, ZrSSe, NbSSe, and PdSSe) towards HCN, H2S, and NH3 using density functional theory (DFT). All monolayers exhibit thermodynamic stability confirmed by negative values of cohesive energy and Gibbs free energy. To probe the electronic properties including the band structure and density of states (DOS) of the nanosheets, HSE06 hybrid functionals were employed. ScSSe and NbSSe show a metallic nature, whereas TiSSe, ZrSSe, and PdSSe display semiconducting behavior with band gaps at 0.147, 1.187 and 1.317 eV. Among the nanosheets, ScSSe, ZrSSe, and NbSSe offer the best balance between adsorption strength and recovery dynamics. NH3 consistently shows strong interactions across all surfaces with significant charge transfer. Recovery time analysis reveals that gas molecules desorb from ScSSe and ZrSSe nanosheets within seconds and milliseconds, respectively, while TiSSe and NbSSe exhibit faster desorption at the microsecond scale. In contrast, PdSSe demonstrates ultrafast recovery in the picosecond range. Therefore, ScSSe, ZrSSe and NbSSe could be preferable candidates for gas sensing operation because of their favorable adsorption energies, electronic properties, quick recovery and high sensitivity.