Oxygen lability on thin oxide films on Mo(110)

The formation and lability of doubly bound, terminal oxygen in thin-film ox ides thermally grown on Mo(110) is studied using reflection-absorption infr ared spectroscopy (RAIRS) and scanning tunneling microscopy (STM) and the i mplications for studies of oxidation reactions on these films is discussed. Isotopic labeling studies show that there is facile exchange of terminal o xygen with oxygen in high coordination sites mediated by defects, even for temperatures on the order of 100 K. The nature and heterogeneity of the Mo= O moieties depends strongly on the temperature used for oxidation. There ar e two different Mo=O species on an oxide prepared at high temperature, 1200 K, signified by vibrational peaks in the range of 995-999 and 1017-1026 cm (-1), attributed to Mo=O moieties on terraces and at steps, respectively. O xidation at lower temperature, 800 K, yields a more homogeneous film based on selective population of the peak in the range of 995-999 cm(-1). The pre sence of the higher frequency peak is associated with formation of multiple steps bunched together on the surface, based on STM studies. The formation of these step bunches is reversible and is related to the amount of oxygen on the surface. Heating so as to diffuse oxygen into the bulk of the sampl e leads to the disappearance of the vibrations characteristic of terminal o xygen. Oxygen diffusion is proposed to occur preferentially at step edges b ased on STM results. The rate of depletion of the terminal oxygen is a diff usive process and has an activation barrier of similar to 0.26 eV. The low barrier is attributed in part to the presence of defects, e.g., steps and o xygen vacancies. Interestingly, a peroxide-like species can also be formed on the oxide by dosing oxygen at low temperature (100 K). This species is s ignified by a band at 900 cm(-1) which shifts to 853 cm(-1) upon O-18 label ing. The adsorbed O-2 dissociates in the range of 200-300 K, forming a term inal site species with a v(Mo=O) at 986 cm(-1).