Selective Gas Sensing Mechanism of Ni/Pd/Pt-doped Janus HfSSe Monolayer Towards H2, CO2, and CH4

Based on first-principles calculations, we systematically investigate the gas sensing behavior of pristine and Ni/Pd/Pt-doped Janus HfSSe monolayers toward H2, CO2, and CH4. Pristine HfSSe exhibits weak physisorption with negligible work-function variation and sensing response. Transition metal doping introduces localized d-orbital states near the Fermi level, which significantly enhance charge transfer and work-function modulation, giving rise to distinct electronic responses for different gas molecules. Specifically, Ni@HfSSe exhibits high sensitivity to CO2, Pd@HfSSe achieves balanced sensitivity and favorable reversibility for CH4, and Pt@HfSSe shows strong H2 response but suffers from poor recovery. Our findings demonstrate that transition metal doping combined with strain engineering provides an effective strategy for tunable and selective gas sensing in Janus two-dimensional materials.