  Recent advances in colloidal synthesis methods have led to increased research
focus on halide perovskites. Due to highly ionic crystal structure of
perovskite materials, stability issue pops up especially against polar solvents
such as water. In this study, we investigate water-driven structural evolution
of CsPbBr3 by performing experiments and state-of-the-art first-principles
calculations. It is seen that while optical image shows the gradual degradation
of yellowish-colored CsPbBr3 structure under daylight, UV illumination reveals
that the degradation of crystals takes place in two steps; transition from
blue-emitting to green-emitting structure and and then transition from
green-emitting phase to complete degradation. We found that as-synthesized
CsPbBr3 NWs emit blue light under 254 nm UV source and before the degradation,
first CsPbBr3 NWs undergoes a water-driven structural transition to form large
bundles. It is also seen that formation of such bundles provide longer-term
environmental stability. In addition theoretical calculations revealed how
strong is the interaction of water molecules with ligands and surfaces of
CsPbBr3 and provide atomistic-level explanation to transition from
ligand-covered nanowires to bundle formation. Further interaction of
green-light-emitting bundles with water causes complete degradation of CsPbBr3
and photoluminescence signal is entirely quenched. Moreover, Raman and XRD
measurements revealed that completely degraded regions are decomposed to PbBr2
and CsBr precursors. We believe that findings of this study may provide further
insight into the degradation mechanism of CsPbBr3 perovskite by water.
