Degradation Chemistry and Kinetic Stabilization of Magnetic CrI₃

The discovery of the intrinsic magnetic order in single-layer chromium trihalides (CrX₃, X = I, Br, and Cl) has drawn intensive interest due to their potential application in spintronic devices. However, the notorious environmental instability of this class of materials under ambient conditions renders their device fabrication and practical application extremely challenging. Here, we performed a systematic investigation of the degradation chemistry of chromium iodide (CrI₃), the most studied among CrX₃ families, via a joint spectroscopic and microscopic analysis of the structural and composition evolution of bulk and exfoliated nanoflakes in different environments. Unlike other air-sensitive 2D materials, CrI₃ undergoes a pseudo-first-order hydrolysis in the presence of pure water toward the formation of amorphous Cr(OH)₃ and hydrogen iodide (HI) with a rate constant of k_I = 0.63 day^-1 without light. In contrast, a faster pseudo-first-order surface oxidation of CrI₃ occurs in a pure O₂ environment, generating CrO₃ and I₂ with a large rate constant of k_Cr = 4.2 day^-1. Both hydrolysis and surface oxidation of CrI₃ can be accelerated via light irradiation, resulting in its ultrafast degradation in air. The new chemical insights obtained allow for the design of an effective stabilization strategy for CrI₃ with preserved optical and magnetic properties. The use of organic acid solvents (e.g., formic acid) as reversible capping agents ensures that CrI₃ nanoflakes remain stable beyond 1 month due to the effective suppression of both hydrolysis and oxidation of CrI₃.