Identification of environment-dependent dominant and metastable doped NiO(110) surfaces

Abstract

Doped NiO-based surfaces (M-NiO) have been extensively explored for diverse catalytic applications due to superior redox properties and tunable structural and electronic properties. Particularly, the less stable yet more reactive NiO(110) facet has the potential to achieve higher catalytic performance. To facilitate the design and in situ control of M-NiO active sites, it is crucial to have a surface-level understanding of the connection between dopant element and environment-dependent surface structure and stability. Here, M-NiO(110) structures were systematically investigated using an integrated ab initio thermodynamic modeling approach combining density functional theory (DFT) and ab initio phase diagrams. The effect of dopant element (Al, Mo, Nb, Sn, Ti, V, W, or Zr), dopant location (surface/subsurface), O vacancies (surface/subsurface), Ni vacancies (surface/subsurface), and adsorbed oxygen species (O*/O₂*) were examined. The dominant NiO(110) structures were the stoichiometric and oxygen-adsorbed surfaces. Introduction of dopants into NiO(110) significantly increased the configurational complexity of the surfaces. Observation of a consistent structural stability between the (110) and (100) facets of M-NiO—latter facet data taken from a previous study—enabled the construction of a linear relation of the surface energies between the two facets and an acceleration of the evaluation of M-NiO(110) structural configurations. Dopants were found to predominantly stabilize the over-oxidized surface structures due to oxophilicity differences between the dopant element and lattice Ni. Furthermore, the presence or absence of adsorbed oxygen species influences the near surface location of the majority of dopants, enabling tuning of surface active sites through environmental treatment conditions of the M-NiO(110) surface. Overall, this work allows for a rapid, effective, and a priori prediction of dominant M-NiO(110) structures with distinct surface structures to potentially facilitate catalytic performance.