Reaction between Pyridine and CH₃NH₃PbI₃: Surface-Confined Reaction or Bulk Transformation?

Pyridine molecules have been used to passivate surface Pb²⁺ sites of CH₃NH₃PbI₃, to recrystallize CH₃NH₃PbI₃, and to bleach CH₃NH₃PbI₃. However, these results contradict each other, as recrystallization and optical-bleach require transformation of bulk CH₃NH₃PbI₃, but surface passivation demands the confinement of the reaction at the surface region. The underlying mechanism for these seemly contradicting results is not yet understood. In this paper, we show, at 25 °C, partial pressure of pyridine vapor is a determining factor for its reaction behaviors with CH₃NH₃PbI₃: one can modify the surface region of CH₃NH₃PbI₃ by using pyridine vapor of pressure 1.15 torr or lower but can transform the whole bulk CH₃NH₃PbI₃ film with a pyridine vapor of 1.3 torr or higher. Our result is the first demonstration that the reaction modes, i.e., surface-confined reaction and bulk transformation, are very sensitive to the partial pressure of under-saturated pyridine vapor. Despite the different reaction behaviors, it is interesting that in all pressure ranges, pyridinium ion is a main product from the reaction between pyridine and CH₃NH₃PbI₃. The bulk transformation is due to the formation of a liquid-like film, which increases the mobility of species to catalyze the reaction between pyridine and CH₃NH₃PbI₃. It is important to note 1.3 torr is much smaller than the saturated vapor pressure of pyridine (20 torr at 25 °C). These findings provide a guidance in applying pyridine and other amines to functionalize and transform CH₃NH₃PbI₃ and other hybrid halide perovskites. It also highlights the critical role of fundamental studies in controllably modifying CH₃NH₃PbI₃.