Probing the Structural Degradation of CsPbBr 3 Perovskite Nanocrystals in the Presence of H 2 O and H 2 S: How Weak Interactions and HSAB Matter.

Structural degradation of all inorganic CsPbBr 3 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, H 2 O and H 2 S, with similar structures, to follow the decomposition mechanism of CsPbBr 3 perovskite nanocrystals. Interestingly, H 2 O acts as a catalyst for the decomposition of CsPbBr 3 , which is in contrast to H 2 S. Our experimental observations followed by density functional theory (DFT) calculations showed that the water molecule is intercalated in the CsPbBr 3 perovskite whereas H 2 S is adsorbed in the (100) planes of CsPbBr 3 by a weak electrostatic interaction. According to Pearson's hard-soft acid-base theory, both cations present in CsPbBr 3 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 H 2 S 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.