Mass transport during transformation of PbI2 infiltrated in nanoporous TiO2 into CH3NH3PbI3 has been investigated by Rutherford backscattering spectroscopy (RBS). Fast initial reaction kinetics were confirmed using optical ex-situ and in-situ measurements. Mapping with energy dispersive X-ray spectroscopy of the cross section of samples revealed a homogeneous PbI2 infiltration in nanoporous TiO2 before transformation but an accumulation of Pb and I at the surface after transformation, in accordance with a depletion of Pb and I in a near surface region. Quantitative depth profiles of Pb and I were obtained from RBS analysis. An instant degradation of CH3NH3PbI3 to PbI2 and volatiles upon ion radiation was found. The concentration profiles of Pb could be simulated with a one dimensional diffusion model taking into account an effective diffusion coefficient of Pb in the nanocomposite (about 1.5 10-11 cm2/s) as well as a parameter considering frazzling at the surface due to formation of crystallites.
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