Covalent Organic Framework-Enhanced Metal Halide Perovskites for Selective and Sensitive Gas Sensing

Solution-processed lead-free halide perovskite gas sensors possess low gas detection limits, offering promising alternatives to traditional metal oxide chemiresistors. However, halide perovskite chemiresistors often suffer from poor selectivity and durability due to a lack of coordinatively unsaturated surface metal ions and their sensitivity to humidity. To address these issues, a general strategy is presented in which the Cs₂PdBr₆ perovskite surface is coated with covalent organic framework (COF) to provide hybrid sensor materials that are highly sensitive to specific gases and demonstrate excellent stability under real-working conditions. The hybrid chemiresistors demonstrate high sensitivity and controllable selectivity toward NO₂ or NH₃ gases. Specifically, TAPB–PDA@Cs₂PdBr₆ achieves a detection limit of 10 ppb for NO₂, the lowest value reported for a perovskite-based gas sensor, maintaining its performance after continuous exposure to ambient air for several weeks. In contrast, COF-5@Cs₂PdBr₆ shows high selectivity to NH₃ and has a detection limit of 40 ppb. Structural and spectroscopic characterization combined with mechanistic studies provide molecular-level insights into the outstanding properties of these new hybrid sensor materials, which set a new benchmark in the field, i.e., surpassing the selectivity and sensitivity of conventional halide perovskite sensors.