Impact of oxygen on the work functions of Mo in vacuum and on ZrO2

The electronic properties of molybdenum surfaces and interfaces with monoclinic zirconia (Mo∕m-ZrO2) of different stoichiometries are investigated through first-principles calculations. We show that oxygen adsorption on the Mo(110) surface strongly increases the Mo vacuum work function, and that a similar trend is observed for the Mo(110) work function on zirconia upon oxygenation of the stoichiometric Mo∕m-ZrO2 interface, albeit to a smaller extent. As expected, Mo∕m-ZrO2 interface reduction/oxidation decreases/increases the Mo effective work function. However, interface overoxidation leading to the formation of a thin MoOx layer between Mo and m-ZrO2 (Mo∕MoOx∕m-ZrO2) causes a work-function decrease with respect to the stoichiometric Mo∕m-ZrO2 interface value. This result is especially surprising because calculations indicate that subsurface oxidation of Mo slabs increases the Mo vacuum work function. Moreover, the calculated vacuum work function of rutile MoO2(110) slab is ∼6.0eV, considerably larger than that of Mo(110) (∼4.9eV). The physical origin of this effect is explained in terms of a partial cancellation interface dipoles.