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 shift 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 pre-treatment 10 and the lower hydrogen partial pressure applied during reduction of the sample.…”

“…The structural changes may be interpreted as a consequence of the release of Ni dopants onto the perovskite surface upon thermal reduction; however, the exsolution response is strongly limited to the surface region. 9,10,18,46 Furthermore, the degree of the decrease in the c -lattice parameter appears not to be directly correlated to the doping level x . In fact, large changes in the lattice parameters are only visible for larger doping levels of x = 0.05 and x = 0.1, while only a minor lattice shrinkage is apparent for thin films with x ≤ 0.03.…”

“…= 458.4 eV in order to compare relative peak shifts between Ti and Ni and to correct for space charge induced peak shifts upon change in ambient atmosphere. 10,48–50 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.…”

“…Furthermore, defect structures and their associated charge and strain fields play a significant role in moderating metal exsolution reactions. 5,8,10,59–64 For instance, the mass transport of exsolution-active dopants to the surface by ionic diffusion results from the concentration gradient that forms between the surface and the sub-surface region, where the exsolution kinetics of surface nanoparticles is strongly entangled with the defect chemistry of the perovskite host lattice. 10 The dynamic formation of space charge regions results in a p (O 2 )-dependant redistribution of the point defect distribution under oxidizing and reducing conditions, which moderates the exsolution reaction.…”

“…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. 8–10 Besides the simplicity of the synthesis concept, the extraordinarily homogeneous distribution and controllable density of the generated exsolution nanoparticles are beneficial for the design of high-performance fuel electrode materials.…”