  The atomic scale insights into the interaction of water with oxide surface
are essential for elucidating the mechanism of physiochemical processes in
various scientific and practical fields, since the water is ubiquitous and
coats multifarious material surface under ambient conditions. By utilizing
periodic density functional theory calculations with van der Waals corrections,
herein we report for the first time the energetically and thermodynamically
favorable homolytic dissociative adsorption behavior of water over magnesia
(001) films deposited on metal substrate. The water adsorption on pristine
magnesia (001) is quite weak and the heterolytic dissociation is the only
fragmentation pathway, which is highly endothermic with large activation
barrier of 1.167 eV. The binding strength for the molecular and dissociative
adsorption configurations of water on hybrid MgO(001) films are significantly
larger than corresponding configurations on bare magnesia (001). The homolytic
dissociative adsorption energy of water on monolayer oxide is calculated to be
-1.192 eV, which is even larger than all the heterolytic dissociative
adsorption states. With the increase of the oxide thickness, the homolytic
dissociative adsorption energy decreases promptly, indicating the nanoscale of
oxide film play a crucial role in homolytic splitting of water. The homolytic
dissociative adsorption structure could be obtained by transformation reaction
from heterolytic dissociative adsorption structure, presenting activation
barriers 0.733, 1.103, 1.306, 1.571, and 1.849 eV for reactions on 1 ML to 5 ML
films. It is anticipated that the results here could provide inspiring clue and
versatile strategy for enhancing chemical reactivity and properties of
insulating oxide toward homolytic water splitting processes involved in novel
and unprecedented applications.
