In situ growth of nanoparticles through control of non-stoichiometry

Abstract

Surfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields, including renewable energy and catalysis. Typically, these structures are prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO 3 stoichiometry. This non-stoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (that is, metallic, oxides or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be influenced strongly by surface reorganization characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application. AbstractSurfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields including renewable energy and catalysis. These structures are typically prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO 3 stoichiometry. This nonstoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (i.e. metallic, oxides, or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be strongly influenced 2 by surface reorganisation characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application.

Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

“…Upon processing at high temperatures required to form most catalytic microstructures, perovskite lattices terminate into A-cation rich layers 31,33,36 which appear to obstruct exsolution 36,87 and interfere negatively with other catalytic perovskite applications, as discussed in section 4.1. This effect is shown in Fig 7a which illustrates that more numerous and better distributed particles exsolve on a freshly cleaved bulk perovskite surface having nominal A-site deficient stoichiometry, as compared to the restructured (native) perovskite surface which is A-cation rich.…”

“…This not only allows for such attractive structural interplay, but also for combining it with other desirable electrode functionality (see Figure 2a) 84,87,89,91,95 , chromite 86,90,94 or chromite-manganite 93,96 perovskite supports. These have been applied to solid oxide fuel cells 86,90,92,94,97 , electrolysis cells 95,98 and for other catalytic process 36,84,85 .…”

“…Such vacancies facilitate ion diffusion by minimising lattice collisions and supplying hopping sites 36 , while creating an apparent B-site excess 87 , which in turn drives the exsolution of catalytically active species from this site in order to retain stable stoichiometry 87 .…”

“…Evidence so far suggests that redox exsolution from perovskites is a phase decomposition process driven by reduction and controlled by bulk and surface defects and external conditions 36,87,92 . Initially the lattice is reduced, losing oxygen and gaining electrons until the point where metal nucleation becomes favourable.…”

“…However, active interfaces can also be generated in situ, during operation, through a phase decomposition process usually referred to as redox exsolution [84][85][86][87] . In this approach, catalytically active transition metals are substituted in a host oxide lattice in oxidising conditions, forming a solid solution, and released (exsolved) on the surface as metal particles, once the oxide lattice has been reduced to sufficient extent.…”