| Abstract | An NMR investigation of methylammonium lead iodide, the leading member of the hybrid organic–inorganic perovskite class of materials, and of its putative decomposition products as a result of exposure to heat and humidity, has been undertaken. We show that the 207Pb NMR spectra of the compound of interest and of the proposed lead-containing decomposition products, CH3NH3PbI3·H2O, (CH3NH3)4PbI6·2H2O, and PbI2, have distinctive chemical shifts spanning over 1400 ppm, making 207Pb NMR an ideal tool for investigating this material; further information may be gained from 13C and 1H NMR spectra. As reported in many investigations of CH3NH3PbI3 on films, the bulk material hydrates in the presence of high relative humidity (approximately 80%), yielding the monohydrated perovskite CH3NH3PbI3·H2O. This reaction is reversible by heating the sample to 341 K. We show that neither (CH3NH3)4PbI6·2H2O nor PbI2 is observed as a decomposition product and that, in contrast to many studies on CH3NH3PbI3 films, the bulk material does not decompose or degrade beyond CH3NH3PbI3·H2O upon prolonged exposure to humidity at ambient temperature. However, exposing CH3NH3PbI3 concurrently to heat and humidity, or directly exposing it to liquid water, leads to the irreversible formation of PbI2. In spite of its absence among the decomposition products, the response of (CH3NH3)4PbI6·2H2O to heat was also investigated. It is stable at temperatures below 336 K but then rapidly dehydrates, first to CH3NH3PbI3·H2O and then to CH3NH3PbI3. The higher stability of the bulk material as reported here is a promising advance, since stability is a major concern in the development of commercial applications for this material. |
|---|
