Kinetics and mechanism of lead-iodide film growth on lead

Abstract

The tarnishing rate of Pb with iodine vapor in the temperature and pressure ranges of 423–523 K and 0.615–6.578 kPa, respectively, have been studied. The film-growth kinetics follow the parabolic law. The iodine-vapor-pressure dependence of the isothermal parabolic rate constant has been observed to be k_P∝p ^1/2 _I2 which is explained on the basis of the migration of electron holes across the film as the rate-limiting step. The activation energy value for iodination of Pb under normal conditions in an iodine pressure of 0.615 kPa is estimated to be 64 kJ·mol⁻¹. In contrast, the rate of iodide-film growth has been found to increase considerably under a short-circuit mode of experiments. Such observations have been explained with the help of ion migration as the rate-limiting step for the film-growth process. The iodine-pressure dependence of the rate constant under short-circuit conditions is found to be k_P∝p ^1/3 _I2 associated with an activation energy of 51 kJ·mol⁻¹. Results of the present study have been explained assuming Schottky-Wagner-type point defects in the lead-iodide film, where an equivalent number of vacancies in the cationic and anionic sublattices are present, and taking into account Wagner's electrochemical potential gradient as the main driving force for the film-growth process. The kinetics results have been substantiated through characterization of iodide films by SEM, EDS, EPMA, and XRD analyses.