Nanopyramidal Reconstruction of Cu<sub>2</sub>O(111): A Long-Standing Surface Puzzle Solved by STM and DFT
Alexander Gloystein, Niklas Nilius, Jacek Goniakowski, Claudine Noguera
Abstract
Despite the importance of cuprous oxide for industrial applications, the precise structure of its most stable reconstructed surface remains unknown. Here, by a combination of high-resolution scanning tunneling microscopy (STM) measurements and simulations based on a density functional theory-HSE approach, we challenge the previous interpretations of the Cu2O(111) (3×3)R30° surface reconstruction based on the removal of one-third of the undercoordinated surface oxygen atoms. We show that these models do not satisfy the shamrock-type features found by STM, that is, their position in the Cu–O ring centers, the orientation toward the surface Ocus atoms, and the topographic height in the STM images. Moreover, they are thermodynamically unstable with respect to moving the vacancy subsurface, a property not recognized before, which seems specific to the Cu2O(111) surface. We propose a nanopyramidal model of the (3×3)R30° surface reconstruction, which is free from all these shortcomings. Here, the shamrock protrusions seen in STM are formed by three copper adatoms located at the center of the Cu–O rings and capped by an oxygen atom. This structure profoundly differs from the existing models of the Cu2O(111) surface and would change common perceptions on its reactivity.