Exploration of the Gas Adsorption/Selection Behavior and Its Doping Regulation Strategy of COFs for Improved Gas-Sensing Performance

To understand the gas-sensing mechanism of COFs and explore an effective modulation way to regulate their sensing properties, the adsorption and sensing behaviors of NO₂, NO, SO₂, O₂, H₂O, CO₂, H₂S, CO, N₂, and NH₃ gas molecules on the surface of pristine and n-doped (Na-adsorption) Tr-Th COFs are explored theoretically with first-principles calculations in this work. Attributed to the lowest unoccupied molecular orbital (LUMO) energy level of NO₂, the adsorption of NO₂ on Tr-Th leads to a larger increase in carrier concentration increment (n = 1.79 × 10¹² to 6.26 × 10¹⁰ cm⁻²) and a greater work function shift (ΔΦ = 0.178 eV) compared to other gases, which suggest that Tr-Th is a highly promising material for NO₂ sensing applications. n-Type doping elevates the Fermi level of COFs, resulting in a greater carrier concentration increment (n = 1.83 × 10¹² cm⁻² ∼ 2.52 × 10¹² cm⁻²) and a larger work function shift (ΔΦ = 0.08 eV ∼ 0.30 eV) upon exposure to NO₂, NO, SO₂, or O₂ compared to other gases. It means that apart from NO₂, NO, SO₂, and O₂ gases will also trap electrons in n-doped Tr-Th COFs, increase the electrical resistance dramatically, and then quench the source leakage current of the COFs-FET gas sensor. Our study provides more detailed information about the gas-sensing mechanism of COFs and highlights the key role that surface doping strategy plays in regulating the gas adsorption and selection behaviors for its practical gas sensor applications.