First-principles study of a penta-PdO2 monolayer: a 2D material for selective detection of toxic gases.

The development of efficient and selective gas-sensing materials is crucial for monitoring toxic atmospheric pollutants and improving environmental safety. In this study, a novel penta-PdO2 monolayer is proposed, and its structural, electronic, optical, and gas-sensing properties are systematically investigated using density functional theory, DFT, calculations. The calculated cohesive energy and phonon dispersion spectra confirm the energetic and dynamical stability of the monolayer, indicating its suitability for practical applications. Pristine penta-PdO2 exhibits semiconducting behavior with an indirect band gap of 0.855 eV. The adsorption characteristics of CO, H2S, NH3, and NO2 gases on the penta-PdO2 surface were analyzed through adsorption energy, charge transfer, electronic structure, and optical response. The results reveal that CO, H2S, and NH3 interact weakly with the surface through physisorption, causing only minor changes in the electronic properties. In contrast, NO2 exhibits strong chemisorption with a large adsorption energy of -0.96 eV and significant charge transfer, indicating strong interaction with the monolayer. NO2 adsorption substantially reduces the band gap and induces spin-polarized magnetic behavior, reflecting pronounced electronic coupling between the gas molecule and the surface. Density of states, electron density difference, and electron localization analyses further confirm the strong orbital hybridization associated with NO2 adsorption. Furthermore, NO2 significantly enhances electrical conductivity and modulates the work function, resulting in a remarkably high sensing response. Recovery time analysis also demonstrates selective retention behavior for NO2. These findings highlight penta-PdO2 as a promising two-dimensional material for selective NO2 adsorption and efficient sensing of CO, H2S, and NH3 gases.