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Combining density-functional-theory calculations and atomistic thermodynamics we study hydrogen adsorption on the Zn-terminated polar ZnO(0001) surface. We extend the concept of equilibrium surface phase diagrams to include kinetically stabilized surface reconstructions by constructing metastable phase diagrams and going beyond the thermodynamic limit. Under these conditions we find a monolayer H coverage to be extremely favorable on triangular-shaped reconstructions due to a simultaneous protonation of step-edge oxygen atoms and surface terminating Zn atoms. Experimental situations, which realize hydrogen chemical potentials outside the stability range of the H₂ molecule are discussed.