Space charge governs the kinetics of metal exsolution

Abstract

Nanostructured composite electrode materials play a major role in the field of catalysis and electrochemistry. Self‐assembly of metallic nanoparticles on oxide supports via metal exsolution relies on the transport of reducible dopants towards the perovskite surface to provide accessible catalytic centers at the solid/gas interface. However, it is unclear if exsolution can be driven from the oxide bulk or if the process is limited to surfaces and interfaces, where strong electrostatic gradients and space charges typically control the properties of oxides. Here we reveal that the nature of the surface‐dopant interaction is the main determining factor for the exsolution kinetics of nickel in SrTi0.9Nb0.05Ni0.05O3‐ẟ and that the exsolution depth is strongly limited to the near‐surface region of the perovskite oxide. Electrostatic interaction of dopants with surface space charge regions forming upon thermal annealing result in strong surface passivation i.e. a retarded exsolution response. We furthermore demonstrate the controllability of the exsolution response via engineering of the perovskite surface chemistry. Our findings indicate that tailoring the electrostatic gradients at the perovskite surface is an essential step to improve exsolution type materials in catalytic converters.

Reversibility limitations of metal exsolution reactions in niobium and nickel co-doped strontium titanate

“…Subsequent reduction of the sample results in a relative shi of the Ni 3p signal towards the Ti 3s peak indicating the formation of metallic surface species, which is correlated to the exsolution of Ni nanoparticles. Notably, the enrichment of Ni at the surface is less pronounced than that demonstrated in our previous study, 51 which can be attributed to the oxidizing pretreatment 10 and the lower hydrogen partial pressure applied during reduction of the sample.…”

“…= 458.4 eV in order to compare relative peak shis between Ti and Ni and to correct for space charge induced peak shis upon change in ambient atmosphere. 10,[48][49][50] First, a mild annealing step in oxygen was performed to desorb carbon species from the surface. The Ni 3p doublet gives rise to a slightly asymmetric peak at higher binding energies relative to the Ti 3s signal.…”

“…Under a reducing thermal treatment, a fraction of surface-near metal dopants is released to the oxide surface where they nucleate in the form of metal nanoparticles, while others remain buried in the oxide matrix. [8][9][10] Besides the simplicity of the synthesis concept, the extraordinarily homogeneous distribution and controllable density of the generated exsolution nanoparticles are benecial for the design of highperformance fuel electrode materials.…”

Ni Ingress and Egress in SrTiO3 Single Crystals of Different Facets

“…However, a certain fraction of the metal is likely to remain in the bulk of the lattice either as dopant atoms/ions or as encapsulated clusters/particles as these might be too deep within the bulk unable to reach the surface . Growth and exsolution from thin oxide films has been proposed as a method to make sure all the doped metal is capable of egress and to form exsolved particles as an alternative route to oxides with bulk thickness. ,,− …”

Enhanced metal exsolution at the non-polar (001) surfaces of multi-faceted epitaxial thin films

“…The effect may be associated with deviations in the surface defect structure that have been shown to play a major role in metal exsolution reactions. For example, defects that form during the synthesis of the parent perovskite or during reducing thermal annealing result in changes in the surface segregation energy [22] as well as in distinct electrostatic interactions between exsolving species and the surface potential that is formed as a consequence of space charge regions that are an inherent feature of oxide surfaces [23,24].…”