Generation of O⁻ Radical Anions on MgO Surface: Long-Distance Charge Separation or Homolytic Dissociation of Chemisorbed Water?

Abstract

O− radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ = 280 and 303 nm. As all corner oxygen atoms are covered with adsorbed hydroxyl groups after activation at 450 °C, this process is initiated by selective excitation of 3-coordinated complexes [Mg2+−O2−]3C containing chemisorbed water. A new mechanism for generation of the O− radical anions is suggested. It is based on homolytic dissociation of chemisorbed water followed by migration of a hydrogen atom, probably, to a different nanoparticle rather than on long-distance separation of charges. The surface structure remaining after its detachment consists of an •OH radical stabilized near the corner oxygen atom. The imminent charge transfer generates a hole at the corner oxygen atom stabilized by a hydroxyl group [OH−···Mg2+−O−]3C. DFT simulation of this structure showed that it reproduces the main characteristics of the radical anions O−3C. The overall structure is electrically neutral. The charge of the hole is compensated by the nearby hydroxyl group. So, no long-distance charge separation is required.