Probing the Structural Degradation of CsPbBr Perovskite Nanocrystals in the Presence of HO and HS: How Weak Interactions and HSAB Matter.

Structural degradation of all inorganic CsPbBr in the presence of moisture is considered as one of its major limitations to use as an active component in various light-harvesting and light-emitting devices. Herein, we used two similar molecules, HO and HS, with similar structures, to follow the decomposition mechanism of CsPbBr perovskite nanocrystals. Interestingly, HO acts as a catalyst for the decomposition of CsPbBr, which is in contrast to HS. Our experimental observations followed by density functional theory (DFT) calculations showed that the water molecule is intercalated in the CsPbBr perovskite whereas HS is adsorbed in the (100) planes of CsPbBr by a weak electrostatic interaction. According to Pearson's hard-soft acid-base theory, both cations present in CsPbBr prefer soft/intermediate bases. In the case of the water molecule, it lacks a soft base and thus it is not directly involved in the reaction whereas HS can provide a soft base and thus it gets involved in the reaction. Understanding the mechanistic aspects of decomposition can give different methodologies for preventing such unwanted reactions.