NO2 Adsorption on Ag(100) Supported MgO(100) Thin Films: Controlling the Adsorption State with Film Thickness

Using photoemission and X-ray absorption spectroscopy, we compare the adsorption properties of NO2 at 300 K on MgO(100)/Ag(100) films with thicknesses varying from 2 to 8 ML and NO2 exposures ranging from 0 L to over 25 000 L. We find that NO2 is stable on 2 ML MgO(100) films, where it is the most abundant adsorbate on the surface (similar to 0.35 ML) for exposures up to at least similar to 25 000 L. At high exposures, NO3 also forms on the surface of 2 ML thick films but is a minority species. In contrast, films thicker than similar to 5 ML show conversion to NO3 beginning already at low exposures. At high exposure to NO2, NO3 is the only species present on the surface. Shifts to lower binding energy of the O 1s spectra with adsorbed species indicate that the NO2 adsorbed on the thin MgO(100) films is likely negatively charged and forms NO2-. A more gradual binding energy shift is observed on thicker films and is likely associated with the slower formation of NO3- Measurements on MgO(1.00) films of various thicknesses indicate that for films thicker than 5 ML, the NO2 adsorption properties are similar and most likely correspond to surfaces of bulk MgO(100). We discuss potential mechanisms for NO2 charging and stabilization on the thin MgO(100) films in the context of recent literature.