Structure and morphology of hydroxylated nickel oxide (111) surfaces

TL;DR

This study combines experimental and theoretical methods to analyze hydroxylated NiO (111) surfaces, revealing structural details, hydroxyl coverage, and the influence of water-driven transitions, with implications for surface morphology and modeling accuracy.

Contribution

It provides a comprehensive analysis of NiO (111) surface reconstructions and hydroxylation, comparing experimental observations with advanced density functional calculations.

Findings

Surfaces contain significant hydroxyl coverage.

Surface structures resemble those of MgO (111).

Octapole structures are unlikely in humid conditions.

Abstract

We report an experimental and theoretical analysis of the sqrt(3)x sqrt(3)-R30 and 2x2 reconstructions on the NiO (111) surface combining transmission electron microscopy, x-ray photoelectron spectroscopy, and reasonably accurate density functional calculations using the meta-GGA hybrid functional TPSSh. While the main focus here is on the surface structure, we also observe an unusual step morphology with terraces containing only even numbers of unit cells during annealing of the surfaces. The experimental data clearly shows that the surfaces contain significant coverage of hydroxyl terminations, and the surface structures are essentially the same as those reported on the MgO (111) surface implying an identical kinetically-limited water-driven structural transition pathway. The octapole structure can therefore be all but ruled out for single crystals of NiO annealed in or transported through humid air. . The theoretical analysis indicates, as expected, that simple density functional theory methods for such strongly-correlated oxide surfaces are marginal, while better consideration of the metal d-electrons has a large effect although, it is still not perfect.