The spontaneous centrosymmetry-breaking and robust room-temperature
ferroelectricity in niobium oxide dihalides spurs a flurry of explorations
into its promising second-order nonlinear optical properties, and
promises potential applications in nonvolatile electro-optical and
optoelectronic devices. However, the ambient stability of the niobium
oxide dihalides remains questionable, which overshadows their future
development. In this work, the chemical degradation of NbOI2 is comprehensively investigated using combined chemical and optical
microscopies in conjunction with spectroscopies. We unveil the highly
anisotropic degradation kinetics of NbOI2 driven by the
hydrolysis process of the unstable dangling iodine bonds dominantly
on the (010) facet and progressing along the c axis. Knowing its degradation mechanism, the NbOI2 flake can then be stabilized by the hexagonal boron nitride encapsulation,
which isolates the air moisture. These findings provide direct insights
into the ambient instability of NbOI2, and they deliver
possible solutions to circumvent this issue, which are essential for
its practical integration in photonic and electronic devices.