Surface Chemical Characterization of Internal Interfaces Generated Within Thin-Film Fe-Ti Hydrides

FeTi is considered an excellent candidate for the reversible storage of hydrogen and has been studied extensively in an attempt to understand the bulk activation needed for this material before use. Segregation of TiO2 to the surface has been noted to occur during activation explaining a slight loss of efficiency per hydride cycle, however, characterization by a host of bulk and surface sensitive techniques has not revealed the cause of this decomposition process.10 nm FeTi samples were prepared in a UHV evaporator both with and without palladium coatings. Post treatment characterization was performed with backscatter conversion electron Mossbauer spectroscopy (CEMS), XPS and SIMS. CEMS is a powerful tool for providing stoichiometric, electronic, magnetic, chemical, and particle size information of iron at depths down to 100 nm. XPS and SIMS are useful to gain quantitative and chemical state information from the topmost 2 nm and the topmost monolayer, respectively. Activation treatments consisted of annealing at 573K and 623K followed by reduction at 573K. Results indicate that ppm levels of H2O in H2 are sufficient to decompose the FeTi alloy and produce TiO2 and Fe metal domains at the surface. Also, at 573K in vacuum, a solid-state reaction was found to occur between Fe oxides and FeTi to produce Fe metal and TiO2. The Pd-FeTi interface was probed with CEMS and the results demonstrate hydrogen dissociation and migration in the absence of alloy decomposition. Our approach uses nondestructive-depth profiling of non-Pd coated FeTi samples along with interfacial information from Pd-FeTi specimens to obtain unique insight into the decomposition process.