Out of the Crystalline Comfort Zone: Sampling the Initial Oxide Formation at Cu(111)

Oxidizing transition metal surfaces are generally characterized by an increasing heterogeneity at simultaneous lowering of crystalline order. This complexity eludes present-day first-principles descriptions, with predictive-quality surface phase diagrams commonly derived from comparing the stability of a small number of ordered surface structural models that are motivated by partial experimental characterization or chemical intuition. Here we leverage the computational acceleration brought by machine-learned interatomic potentials for a systematic sampling of the configurational phase space through replica exchange molecular dynamics. Thermodynamic averaging subsequently yields grand-canonical expectation values for observables like O coverage that account for the disorder and diversity of the sampled structures. Application to the initial oxidation of the Cu(111) surface reveals the (purely entropic) stabilization of sparse O adsorbates at the onset, a plethora of energetically essentially degenerate polymeric –O–Cu–O– ring and chain networks at higher O loading, as well as the presence of experimentally discussed minority species. The in silico surface phase diagram correspondingly shows marked differences to one based merely on established ordered surface reconstructions.