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 r ange 500-700 K to form films that consist predominantly of iron chlori de. Two growth regimes are found: (1) parabolic, where the film thickn ess (X) varies as a function of time as X(2) similar to t and where th e growth rate is independent of pressure, and (2) a linear growth regi on (X similar to t), where the growth kinetics are first order in CCl4 pressure. The kinetics are analyzed by a model which assumes that-gro wth is controlled either by CCl4 thermally decomposing at the growing gas-film interface or by diffusion through the film. Analysis of the e xperimental 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 chlor ide and chloroform, where small particles of carbon were identified in the film using Raman spectroscopy, no carbon particles are found in f ilms formed from carbon tetrachloride decomposition below similar to 7 00 K although carbon is found in the film using X-ray photoelectron sp ectroscopy. XPS results also suggest that, in addition to completely t hermally decomposing to yield carbon and chlorine, a small portion of the CCl4 can react to form adsorbed CCl2 species.