“Fight Fire with Fire”: Trace‐Water‐Induced Controllable Synthesis of Hydrophobic Armors to Stabilize Metal‐Organic Cage‐Based Crystalline Frameworks

Abstract Metal‐organic cage‐based crystalline frameworks (MCFs) are distinguished for high porosity and diverse functionality, while their applications are constrained by degradation in wet environments. Inspired by the “fight fire with fire” method in traditional Chinese medicine, trace‐water‐induced synthesis of armors is proposed to stabilize MCFs. Water at ppm concentration is enriched on the hydrophilic surface of MCFs, and then polymerizes with diisocyanate under the catalysis of MCFs to form hydrophobic shells. The moderate polymerization conditions render this approach extendable to various fragile MCFs. The hydrophobic shells endow the MCF‐BPDC@PHDI derived from the polymerization of hexamethylene diisocyanate (HDI) onto MCF‐BPDC (BPDC═ biphenyl‐4,4’‐dicarboxylic acid) with improved adsorption capacity and stability in wet environments, making it one of the best materials for iodine adsorption. Moreover, the armored MCF can adsorb iodine species at ultra‐low concentrations and capture anionic iodine selectively from competitive cations for the first time. This study might open up an avenue for controllable fabrication of armors by using “adverse” water, allowing the stabilization of various materials.