Control of morphology of covalent organic framework-5 for chemiresistive low-temperature ammonia sensors

Covalent organic frameworks (COFs) have attracted much attention due to their unique structures and properties. However, the morphology regulation of COFs has always been a challenge, and the application field of COFs need to be further explored. Herein, three different regular morphologies of COF-5 crystals, including nanoparticles, nanorods and nanosheets, are firstly regulated by adjusting the reaction parameters under the convenient and mild conditions. The molecular configuration and pore size distribution of the as-prepared COF-5 crystals were revealed by DFT calculation and experimental tests. The ammonia sensing test proves that all three COF-5 crystals with different morphologies have good response at room temperature, and COF-5 nanosheets with maximum specific surface area possess the lowest detection limit of 600 ppb towards ammonia gas. Furthermore, COF-5 crystals-based gas sensors have excellent reversibility, gas selectivity and long-term stability. Gas sensing mechanism illustrates that abundant boronate esters with Lewis acidity of COF-5 crystals act as the active site to realize the ammonia sensitive detection.