Degradation Chemistry and Kinetic Stabilization of Magnetic CrI 3 .

The discovery of the intrinsic magnetic order in single-layer chromium trihalides (CrX 3 , 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 (CrI 3 ), the most studied among CrX 3 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, CrI 3 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 CrI 3 occurs in a pure O 2 environment, generating CrO 3 and I 2 with a large rate constant of k Cr = 4.2 day -1 . Both hydrolysis and surface oxidation of CrI 3 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 CrI 3 with preserved optical and magnetic properties. The use of organic acid solvents ( e.g. , formic acid) as reversible capping agents ensures that CrI 3 nanoflakes remain stable beyond 1 month due to the effective suppression of both hydrolysis and oxidation of CrI 3 .