Surface structure of magnetite (111) under oxidizing and reducing conditions

We report on differences in the magnetite (111) surface structure when prepared under oxidizing and reducing conditions. Both preparations were done under UHV conditions at elevated temperatures, but in one case the sample was cooled down while keeping it in an oxygen atmosphere. Scanning tunneling microscopy after each of the preparations showed a different apparent morphology, which is discussed to be an electronic effect and which is reflected in the necessity of using opposite bias tunneling voltages in order to obtain good images. Surface x-ray diffraction revealed that both preparations lead to Fe vacancies, leading to local O-terminations, the relative fraction of which depending on the preparation. The preparation under reducing conditions lead to a larger fraction of Fe-termination. The geometric structure of the two different terminations was found to be identical for both treatments, even though the surface and near-surface regions exhibit small compositional differences; after the oxidizing treatment they are iron deficient. Further evidence for the dependence of iron vs oxygen fractional surface terminations on preparation conditions comes from Fourier transform infrared reflection-absorption spectroscopy, which is used to study the adsorption of formic acid. These molecules dissociate and adsorb in chelating and bidentate bridging geometries on the Fe-terminated areas and the signal of typical infrared absorption bands is stronger after the preparation under reducing conditions, which results in a higher fraction of Fe-termination. The adsorption of formic acid induced an atomic roughening of the magnetite (111) surface which we conclude from the quantitative analysis of the crystal truncation rod data. The roughening process is initiated by atomic hydrogen, which results from the dissociation of formic acid after its adsorption on the surface. Atomic hydrogen adsorbs at surface oxygen and after recombination with another H this surface hydroxyl can form H₂O, which may desorb from the surface, while iron ions diffuse into interstitial sites in the bulk.