Revisiting The Origin Of Low Work Function Areas In Pattern Forming Reaction Systems: Electropositive Contaminants Or Subsurface Oxygen?

In pattern-forming reaction systems, the conversion of macroscopic islands of chemisorbed oxygen into low work function (WF) areas has been attributed to formation of a subsurface oxygen species. We prepare micrometer-sized oxygen islands on Pt(100) and Pt(110) by reaction fronts in catalytic CO oxidation in the 10(-6) mbar range. By applying electron microscopy with chemical and structural sensitivity, we characterize their chemical composition upon annealing in vacuum. On Pt(100) we reproduce the previously reported transformation from high to low WF, estimating a relative change Delta phi approximate to-0.8 eV with respect to the CO covered surface. We demonstrate that the change in WF is due to a strong enrichment of electropositive contaminants, namely Ca and Ti, in the oxygen islands. On Pt(110) predosed with Cs, we find that the alkali metal accumulates in the oxygen islands, producing Delta phi approximate to -0.4 eV relative to CO-adlayer. Our experiments suggest that the interpretation of the low WF areas as due to "subsurface oxygen" should be revised and attributed to the local enrichment of electropositive species.