DFT study of water adsorption and decomposition on a Ga-rich GaP(001)(2×4) surface

We investigate the adsorption and decomposition states of a water molecule on a Ga-rich GaP(001)(2×4) surface using the PBE flavor of density functional theory (DFT). We selected the GaP(001)(2×4) mixed dimer surface reconstruction model to represent the Ga-rich GaP(001)(2×4) surface. Because our focus is on reactions between a single water molecule and the surface, the surface water coverage is kept at 0.125 ML, which corresponds to one water molecule in the (2×4) unit cell. We report here the geometries and energies for an exhaustive set of adsorption and decomposition states induced by a water molecule on the (2×4) unit cell. Our results support a mechanism in which (1) the first step is the molecular adsorption, with the water molecule forming a Lewis acid-Lewis base bond to the sp ² Ga atom of either the first-layer Ga-P mixed dimer or the second layer Ga-Ga dimers using an addition reaction, (2) which is followed by dissociation of the adsorbed H ₂O to form the HO/H decomposition state in which the hydroxyl moiety bonds with surface sp ² Ga atoms, while the hydrogen moiety binds with the first-layer P atom, (3) which is followed by the O/2H decomposition state, in which the oxygen moiety forms bridged Ga-O-Ga structures with surface Ga dimers while one H bonds with the first-layer P atom and the other to surface sp ² Ga atoms. (4) We find that driving off the hydrogen as H ₂ leads to the surface oxide state, bridged Ga-O-Ga structures. This surface oxide formation reaction is exothermic relative to the energy of H ₂O plus the reconstructed surface. These results provide guidelines for experiments and theory to validate the key steps and to obtain kinetics data for modeling the growth processes.