Structure and growth kinetics of films formed by the thermal decomposition of CCl₄ on iron surfaces

Carbon tetrachloride thermally decomposes on iron in the temperature range 500-700 K to form films that consist predominantly of iron chloride. Two growth regimes are found: (1) parabolic, where the film thickness (X) varies as a function of time as X² ∼ t and where the growth rate is independent of pressure, and (2) a linear growth region (X ∼ t), where the growth kinetics are first order in CCl₄ pressure. The kinetics are analyzed by a model which assumes that growth is controlled either by CCl₄ thermally decomposing at the growing gas-film interface or by diffusion through the film. Analysis of the experimental data gives an activation energy for diffusion of 21.5 ± 0.3 kcal/mol and an activation energy for CCl₄ decomposition of 18.3 ± 0.5 kcal/mol. In contrast to the behavior found for methylene chloride and chloroform, where small particles of carbon were identified in the film using Raman spectroscopy, no carbon particles are found in films formed from carbon tetrachloride decomposition below ∼700 K although carbon is found in the film using X-ray photoelectron spectroscopy. XPS results also suggest that, in addition to completely thermally decomposing to yield carbon and chlorine, a small portion of the CCl₄ can react to form adsorbed CCl₂ species.