Chemistry at corners and edges: Generation and adsorption of H atoms on the surface of MgO nanocubes

We used UV light to generate site-selective ${\mathrm{O}}^{-}$ hole centers at three-coordinated corner oxygen sites on MgO nanocubes. These highly reactive ${\mathrm{O}}^{-}$ radicals split ${\mathrm{H}}_2$ homolytically and, in the course of this reaction, become hydroxylated and produce hydrogen atoms. The hydrogen atoms adsorb predominantly at cube edges and dissociate into surface-trapped electrons and protons. We propose that the experimentally observed $\left({\mathrm{H}}^{+}\right)\left(e^{-}\right)$ centers are formed adjacent to the hydroxyl groups generated in the homolytic splitting process and can be defined as ${\left({\mathrm{H}}^{+}\right)}_{3\mathrm{C}}\cdots \left(e^{-}\right){\left({\mathrm{H}}^{+}\right)}_{\mathrm{N}\mathrm{C}}$ centers where 3C and NC refer to the coordination numbers of the corresponding hydroxylated oxygen sites. Our ab initio embedded cluster calculations reveal that the electronic properties of ${\left({\mathrm{H}}^{+}\right)}_{3\mathrm{C}}\cdots \left(e^{-}\right){\left({\mathrm{H}}^{+}\right)}_{4\mathrm{C}}$ centers situated along MgO nanocube edges are consistent with both the electron-paramagnetic-resonance signal parameters and the reported optical-absorption properties. The transformation of corner ${\mathrm{O}}^{-}$ centers into the ${\left({\mathrm{H}}^{+}\right)}_{3\mathrm{C}}\cdots \left(e^{-}\right){\left({\mathrm{H}}^{+}\right)}_{\mathrm{N}\mathrm{C}}$ -type centers prevents their recombination with electronic surface centers and, hence, significantly alters the electronic structure of MgO nanocubes by introducing shallow electron traps.