Competition between clustering and dispersion of cobalt atoms on perovskite surfaces: SrTiO3(001) and KTaO3(001)

TL;DR

This study investigates how cobalt atoms interact with and distribute on SrTiO3 and KTaO3 perovskite surfaces, revealing competing clustering and incorporation mechanisms that depend on surface chemistry and annealing conditions.

Contribution

It provides an atomic-scale understanding of cobalt behavior on perovskite surfaces, combining microscopy and spectroscopy to reveal mechanisms of clustering and incorporation.

Findings

Cobalt prefers ionic states (+2 and +3) on surfaces.

Cobalt remains dispersed as single atoms up to certain conditions.

Annealing induces clustering and surface incorporation of cobalt.

Abstract

Perovskite oxides are attractive for reactions in photo/electrocatalytic schemes, and extrinsic doping is a common strategy for tuning their properties. It is widely known that extrinsic dopants impact the structure and stability of perovskite surfaces, but an atomic-scale view is missing. Here, noncontact atomic force microscopy (ncAFM) and photoelectron spectroscopy (XPS/PES) are used to combine microscopic and spectroscopic evidence of cobalt adsorption, incorporation, and clustering at surfaces of two prototypical perovskites SrTiO3 and KTaO3. A number of different sub-ML coverages and temperatures of annealing were investigated. Several common features are observed: cobalt shows a strong preference for ionic nature (+2 and +3 charge states), and remains dispersed as single atoms to a certain extent in both perovskites. Two competing mechanisms are observed upon annealing: coalescence into clusters with a mixed metallic/ionic character, and incorporation into the surface and subsurface regions. The latter is more pronounced in SrTiO3, where a cobalt-stabilized surface reconstruction is identified, whereas for KTaO3 cobalt likely incorporates in the near-surface region.