Imaging the facet surface strain state of supported multi-faceted Pt nanoparticles during reaction

Abstract

Nanostructures with specific crystallographic planes display distinctive physico-chemical properties because of their unique atomic arrangements, resulting in widespread applications in catalysis, energy conversion or sensing. Understanding strain dynamics and their relationship with crystallographic facets have been largely unexplored. Here, we reveal in situ, in three-dimensions and at the nanoscale, the volume, surface and interface strain evolution of single supported platinum nanocrystals during reaction using coherent x-ray diffractive imaging. Interestingly, identical {hkl} facets show equivalent catalytic response during non-stoichiometric cycles. Periodic strain variations are rationalised in terms of O₂ adsorption or desorption during O₂ exposure or CO oxidation under reducing conditions, respectively. During stoichiometric CO oxidation, the strain evolution is, however, no longer facet dependent. Large strain variations are observed in localised areas, in particular in the vicinity of the substrate/particle interface, suggesting a significant influence of the substrate on the reactivity. These findings will improve the understanding of dynamic properties in catalysis and related fields.

Fig. 1. Experimental set-up and morphology of Pt nanocrystal.

a Photograph of the experimental set-up with the gas reactor at the P10 beamline of the PETRA III synchrotron. b–f Different views of the BCDI reconstruction (drawn at 50% of the maximum Bragg electron density) of NP300. The different facets are indexed by their Miller (hkl) indices. The colour coding in Fig. b varies as a function of the facet size. The colour coding in Fig. c–f varies with the value of the normal of the facets. The position of the substrate with respect to the NP is shown in b, showing the [1 1 1] out-of-plane orientation of the Pt NPs.

Fig. 2. In situ 3D lattice displacement images during reaction for NP300.

Evolution of BCDI reconstructions of the displacement field and phase along the [111] direction, drawn at 50% (see Supplementary Fig. S6 for the choice of this value for the isosurface) of the maximum Bragg electron density of NP300 at 450 ^∘C and for different gas mixtures (CO oxidation reaction under stoichiometric conditions is indicated in bold). The left (right) panel, which corresponds to the first (last) measurement, shows the particle at the beginning (end) of the corresponding gas mixture: a Ar, b Ar + 2.5% O₂, c Ar, 2.5% O₂ + 12.5% CO, d Ar, e Ar + 2.5% O₂, f Ar + 2.5% O₂ + 25% CO, g Ar + 12.5% O₂, h Ar + 12.5% O₂ + 25% CO, i Ar + 25% CO, j Ar and k Ar + 12.5% O₂. The “last” measurement has been taken after reaching the saturation of the changes. It corresponds to the steady state of the particle. Note that the crystal is represented in the laboratory frame, which explains the observed 20^∘ tilt corresponding to the incidence angle of the incoming x-ray beam. In addition, because of the representation in the laboratory frame, X, Y, and Z do not correspond to any particular crystallographic direction. The direction of the scattering vector q₁₁₁ from several points of view is indicated for some images of the top row (condition a). During condition f only a single measurement was taken and is thus displayed.

Fig. 3. In situ structural evolution of NP300 during reaction.

Evolution of (a) the full width at half maximum (FWHM) of its diffraction pattern along the Q_y direction, of (b) its average strain, 〈ϵ₁₁₁〉 calculated from the centre of mass of the diffraction patterns or from the reconstructed strain field, of (c) its strain field energy and of (d) the gas composition at the inlet (Ar, O₂, and CO are in orange, green and purple, respectively) as a function of time. The dashed line indicates the beginning of the CO oxidation reaction under stoichiometric conditions.

Fig. 4. Comparison between experiment and simulation for NP650.

a, c 200 nm simulated NP with the same faceting as NP650 and relaxed by energy minimisation seen from two different field of views (b), (d) Experimental ϵ₁₁₁ strain field measured in Ar atmosphere for NP650 and seen from the same field of views.

Fig. 5. Strain evolution during O₂ adsorption and CO oxidation for NP300.

a–d 1st cycle: a Ar, b Ar + O₂: 2.5% (14 min. exposure), c Ar + O₂: 2.5% (59 min. exposure) and d Ar + CO: 25% + O₂: 2.5% (10 min. exposure). e–h 2nd cycle: e Ar, f Ar + O₂: 2.5% (10 min. exposure), g Ar + O₂: 2.5% (76 min. exposure) and h Ar + CO: 25% + O₂: 2.5% (19 min. exposure). i–l 3rd cycle: i Ar + O₂: 12.5% (70 min. exposure), j Ar + CO: 25% + O₂: 12.5% (21 min. exposure), k Ar + CO: 25% + O₂: 12.5% (53 min. exposure) and l Ar + O₂: 12.5% (13 min. exposure). The colour coding of the Miller indices denotes the variation of the average strain per facet Δ〈ϵ₁₁₁〉 with respect to the previous state.

Fig. 6. Comparison of the average strain evolution (Δϵ₁₁₁) per {hkl} facet subfamily during O₂ adsorption and CO oxidation.

The strain variation is given between two consecutive gas conditions. a NP300, first two cycles (χ = 10). b NP650 (χ = 10) and (χ = 2). A yellow background indicates a switch from pure Ar to Ar + O₂ while a blue background indicates a switch from Ar + O₂ to CO oxidation reaction conditions.

Fig. 7. Facet analysis of the displacement field for NP300.

Evolution of the average displacement field for three selected facets of the four different facet subfamilies: a {100}, b {1$\overline{1}$0}, c {1$\overline{1}$1} and d {11$\overline{3}$} facets. The evolution of three facets are shown with at least one of them experiencing large variations during the CO oxidation and one of them unaffected by the reaction. The error bars represent the standard deviation of the displacement per facet. The large inhomogeneous displacement observed during the stoichiometric CO oxidation reaction is reflected by large error bars.