${\mathrm{CsPbBr}}_3$ perovskites: Theoretical and experimental investigation on water-assisted transition from nanowire formation to degradation

Recent advances in colloidal synthesis methods have led to an increased research focus on halide perovskites. Due to the highly ionic crystal structure of perovskite materials, a stability issue pops up, especially against polar solvents such as water. In this study, we investigate water-driven structural evolution of ${\mathrm{CsPbBr}}_3$ by performing experiments and state-of-the-art first-principles calculations. It is seen that while an optical image shows the gradual degradation of the yellowish ${\mathrm{CsPbBr}}_3$ structure under daylight, UV illumination reveals that the degradation of crystals takes place in two steps: transition from a blue-emitting to green-emitting structure and and then a transition from a green-emitting phase to complete degradation. We found that as-synthesized ${\mathrm{CsPbBr}}_3$ nanowires (NWs) emit blue light under a 254 nm UV source. Before the degradation, first, ${\mathrm{CsPbBr}}_3$ NWs undergo a water-driven structural transition to form large bundles. It is also seen that formation of such bundles provides longer-term environmental stability. In addition theoretical calculations revealed the strength of the interaction of water molecules with ligands and surfaces of ${\mathrm{CsPbBr}}_3$ and provide an atomistic-level explanation to a transition from ligand-covered NWs to bundle formation. Further interaction of green-light–emitting bundles with water causes complete degradation of ${\mathrm{CsPbBr}}_3$ and the photoluminescence signal is entirely quenched. Moreover, Raman and x-ray-diffraction measurements revealed that completely degraded regions are decomposed to ${\mathrm{PbBr}}_2$ and CsBr precursors. We believe that the findings of this study may provide further insight into the degradation mechanism of ${\mathrm{CsPbBr}}_3$ perovskite by water.

Figure 1

Appearance of ${\mathrm{CsPbBr}}_3$ under daylight and UV illumination; (a) initially after casting and (b) after 24 and (c) 144 h treated with water, respectively. Focused region in (c) presents the optical microscope image of the related sample under daylight and UV excitation.

Figure 2

(a) Crystal structure, (b) SEM image, (c) Raman measurement (inset: XRD pattern), and (d) photoluminescence and absorption spectra (inset: photograph under 254 nm UV light) of ${\mathrm{CsPbBr}}_3$ NWs.

Figure 3

(a) Time-dependent photoluminescence of as-synthesized NWs interacting with water, (b) SEM image of the resulting NW structure after 24 h of interaction, and (c)–(d) XRD measurement and Raman spectra of both as-synthesized and water-interacted samples. Scale bars are $2\mu \mathrm{m}$.

Figure 4

Side views of (a) Cs-terminated and (b) Pb-terminated ${\mathrm{CsPbBr}}_3$ surface.

Figure 5

(a) PL measurement of ${\mathrm{CsPbBr}}_3$ NWs to degraded crystals, (b) optical image from nonemitting bundles, and (c) XRD pattern and (d) Raman measurement of ${\mathrm{CsPbBr}}_3$ NWs, degraded crystals, CsPb, and ${\mathrm{PbBr}}_2$ powder. Scale bar is $50\mu \mathrm{m}$.