Squeezing Out Nanoparticles from Perovskites: Controlling Exsolution with Pressure

Abstract

Nanoparticle exsolution has emerged as a versatile method to functionalize oxides with robust metallic nanoparticles for catalytic and energy applications. By modifying certain external parameters during thermal reduction (temperature, time, reducing gas), some morphological and/or compositional properties of the exsolved nanoparticles can be tuned. Here, it is shown how the application of high pressure (<100 bar H₂) enables the control of the exsolution of ternary FeCoNi alloyed nanoparticles from a double perovskite. H₂ pressure affects the lattice expansion and the nanoparticle characteristics (size, population, and composition). The composition of the alloyed nanoparticles could be controlled, showing a reversal of the expected thermodynamic trend at 10 and 50 bar, where Fe becomes the main component instead of Ni. In addition, pressure drastically lowers the exsolution temperature to 300 °C, resulting in unprecedented highly-dispersed and small-sized nanoparticles with a similar composition to those obtained at 600 °C and 10 bar. The mechanisms behind the effects of pressure on exsolution are discussed, involving kinetic, surface thermodynamics, and lattice-strain factors. A volcano-like trend of the exsolution extent suggests that competing pressure-dependent mechanisms govern the process. Pressure emerges as a new design tool for metallic nanoparticle exsolution enabling novel nanocatalysts and surface-functionalized materials.

Keywords: double perovskites; exsolution; metallic nanoparticles; pressure; ternary alloys.

Figure 1

Schematic representation of the different treatment parameters that can be used to modify morphological and/or compositional properties of the exsolved NPs. The increasing temperature usually tailors NP growth, but also compositional changes when involving alloyed exsolution. Longer exposure times to thermal reduction treatment mainly lead to higher NP populations, whereas alternative reducing atmosphere to H₂ can trigger NP shape modifications. In this work, the effect of pressure during the exsolution process is evaluated, and a remarkable impact on alloyed NPs composition is observed, together with the possibility of lowering exsolution treatment requirements.

Figure 2

Powder X‐ray diffraction patterns for Sr₂FeCo_0.2Ni_0.2Mn_0.1Mo_0.5O_6‐δ before and after reduction treatments at 600 °C, 2 h under H₂ flow. Different treatment pressures were tested (1, 10, 50, and 100 bar). An additional phase is formed at 10 and 50 bar, and the Sr₃FeMoO₇ Ruddlesden‐Popper phase shows up (red dots). a) Some metallic‐phase signals can be appreciated after treatment at 10 bar (green triangles). HRFESEM micrographs of Sr₂FeCo_0.2Ni_0.2Mn_0.1Mo_0.5O_6‐δ after exsolution treatments at 600 °C, 2 h under H₂ flow and different pressures: b) 1, c) 10, d) 50, and e) 100 bar. f) NPs size and populations change with different pressure treatments.

Figure 3

HAADF‐STEM and map images of Sr₂FeCo_0.2Ni_0.2Mn_0.1Mo_0.5O_6‐δ after 600 °C, 2 h exsolution treatments at a) 1 and b) 10 bar. Both treatments led to the exsolution of ternary alloyed FeCoNi NPs. c) Elemental composition of the exsolved NPs at different treatment pressures. HRTEM micrographs after exsolving 2 h at 600 °C for d) 1 bar, e) 10 bar, f) 50 bar, and g) 100 bar treatments and their corresponding digital diffraction patterns (DDP), showing interplanar distances corresponding to (111) planes.

Figure 4

a) Powder diffraction patterns for Sr₂FeCo_0.2Ni_0.2Mn_0.1Mo_0.5O_6‐δ before and after reduction treatments at 300 °C, 2 h under H₂ flow. Different treatment pressures were tested (1, 10, 50 and 100 bar). HRFESEM micrographs after reduction treatments at 300 °C, 2 h at b) 1 and c) 10 bar of pressure. No exsolved NPs can be seen at atmospheric pressure, but remarkable exsolution occurs by applying 10 bar. d) HRTEM micrograph of an NP after exsolving 2 h at 300 °C and 10 bar and e) the corresponding digital diffraction pattern (DDP). HRFESEM micrographs after reduction treatments at f) 300 °C, 2 h at 50, and g) 100 bar.

Figure 5

a) Variation of crystallographic parameters of the double perovskite and the exsolved NPs at different pressures for 600 °C treatments. b) Observed trend for DP lattice parameter when exsolving at 600 and 300 °C. c) Theoretical nucleation rate variation with pressure at the studied exsolution temperatures (300 and 600 °C).

Figure 6

Schematic explanation of pressurized‐reduction effects over exsolution and crystal phase impact on Sr₂FeCo_0.2Ni_0.2Mn_0.1Mo_0.5O_6‐δ. Volcano‐plot‐like influence of pressure is considered, based on the experimental results presented here, which indicate two main competing effects: enhanced chemical reduction due to higher H₂ collisions, thus more oxygen vacancies are created; and compressive strain. Due to visual reasons, the volcano‐like plot is represented symmetrically. Nevertheless, our experimental results (see Figure 3e) show that the exsolution extent may reach an optimal value between 10 and 50 bar treatments, so the volcano‐like trend would be asymmetric. A‐site atoms (Sr) are represented in green; B‐site atoms (Fe, Co, Ni, Mn) in brown; B‐site Mo in purple; and O atoms in red in the crystal structures. Compressive strain is represented with red arrows, free space for cation diffusion reduction, with black arrows.