NO ₂ 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 NO ₂ exposures ranging from 0 L to over 25 000 L. We find that NO ₂ is stable on 2 ML MgO(100) films, where it is the most abundant adsorbate on the surface (∼0.35 ML) for exposures up to at least ∼25 000 L. At high exposures, NO ₃ also forms on the surface of 2 ML thick films but is a minority species. In contrast, films thicker than ∼5 ML show conversion to NO ₃ beginning already at low exposures. At high exposure to NO ₂, NO ₃ is the only species present on the surface. Shifts to lower binding energy of the O 1s spectra with adsorbed species indicate that the NO ₂ adsorbed on the thin MgO(100) films is likely negatively charged and forms NO ₂-. A more gradual binding energy shift is observed on thicker films and is likely associated with the slower formation of NO ₃-. Measurements on MgO(100) films of various thicknesses indicate that for films thicker than 5 ML, the NO ₂ adsorption properties are similar and most likely correspond to surfaces of bulk MgO(100). We discuss potential mechanisms for NO ₂ charging and stabilization on the thin MgO(100) films in the context of recent literature.