Synthetic control over lattice strain in trimetallic AuCu-core Pt-shell nanoparticles

Abstract

Core-shell nanoparticles can exhibit strongly enhanced performances in electro-, photo- and thermal catalysis. Lattice strain plays a key role in this and is induced by the mismatch between the crystal structure of the core and the shell metal. However, investigating the impact of lattice strain has been challenging due to the lack of a material system in which lattice strain can be controlled systematically, hampering further progress in the field of core-shell catalysis. In this work, we achieve such a core-shell nanoparticle system through the colloidal synthesis of trimetallic Pt-shell Au_1-xCu_x-core nanoparticles. Our seed-mediated growth methodology yields well-defined Au_1-xCu_x-cores, tunable in composition from 0 at% Cu to 77 at% Cu, and monodisperse in size. Subsequent overgrowth results in uniform, epitaxially grown Pt-shells with a controlled thickness of ∼3 atomic layers. By employing a multi-technique characterization strategy combining X-ray diffraction, electron diffraction and aberration corrected electron microscopy, we unravel the atomic structure of the trimetallic system on a single nanoparticle-, ensemble- and bulk scale level, and we unambiguously demonstrate the controlled variation of strain in the Pt-shell from -3.62% compressive-, to +3.79% tensile strain, while retaining full control over all other structural characteristics of the system.

Fig. 1. Overview of the 3-step colloid synthesis procedure used to prepare trimetallic Pt-shell Au_1−xCu_x-cores. (a) Schematic overview of the applied synthesis 3-step synthesis procedure, and bright-field TEM images of (b) 11.5 ± 2.9 nm Au nuclei, (c) 12.7 ± 2.0 nm Au_0.62Cu_0.38 nanoparticles and (d) 15.3 ± 2.3 nm Pt/Au_0.62Cu_0.38 core–shell nanoparticles.

Fig. 2. Overview of the core–shell metal distribution on both individual nanoparticle and ensemble level of a typical synthesized Pt/Au_1−xCu_x nanoparticle system. (a) Energy dispersive X-ray spectroscopy (EDX) map of a single Pt-shell AuCu-core nanoparticle indicating a core–shell metal distribution. (b) Linescan of EDX signal intensities along the white arrow drawn in (a), showing the presence of gold and copper in the core and platinum in the shell. Red, green & blue indicate gold, copper & platinum, respectively. The Pt signal was primarily located near the edge of the particle, whereas the Au and Cu signals were primarily observed in the core. (c) EDX map of Pt/Au_0.59Cu_0.41 nanoparticles with an average size of 15.4 ± 2.0 nm. (d) Intensity correlation plot of the Pt signal (blue) with Au signal red from (c). The brighter the observed spot, the stronger the correlation that was observed for the two signals. (e) Intensity correlation plot of the Au signal (red) with the Cu signal (green).

Fig. 3. Overview of STEM-EDX investigation into the differences between individual nanoparticle compositions of three Pt/Au_1−xCu_x samples with increasing Cu core content and constant Pt composition. Here, x = 0.19 for (a–c), x = 0.55 for (d–f) and x = 0.77 for (g–i) show STEM images, corresponding EDX maps in (b, e and h) and individual particle compositions in (c, f and i). Red, green & blue indicate gold, copper & platinum, respectively.

Fig. 4. Local assessment of nanoparticle lattice parameter as a function of core Cu at%. (a) TEM image of representative area used for selected area electron diffraction. (b) Corresponding electron diffractogram at a camera length of 520 mm. The dots indicate defined crystal planes found in individual nanoparticles. (c) Azimuthally integrated intensities of 2D electron diffractograms for Pt/Au_1−xCu_x ranging from x = 0.19 to x = 0.77. The grey dotted line represents the scattering vector for the Au{200} plane family. The scattering vector increased as more copper was incorporated into the core. A vertical offset was introduced to enhance visibility, while the scattering vector remained unchanged. (d) Experimental lattice parameters plotted as function of core copper content. The lattice parameter decreased as more copper was incorporated into the core. Lattice parameters deviated approximately 1% from the corresponding literature XRD values, which are represented by the dashed black line.

Fig. 5. Bulk scale assessment of nanoparticle lattice parameter using X-ray diffraction as a function of Cu at%. (a) X-ray diffraction data for Pt/Au_1−xCu_x ranging from x = 0.19 to x = 0.77. The grey dotted line represents the scattering vector for the Au{200} plane family. The scattering vector increased with increasing Cu atm%. (b) Calculated lattice parameter plotted as function of core copper content. Dashed black line corresponds to XRD literature values for the Au–Cu system. In Fig. S6, the XRD patterns for Pt/Au_0.45Cu_0.55 and Pt/Au_0.23Cu_0.77 are shown with appropriate reference patterns.

Fig. 6. Aberration corrected high-resolution transmission electron microscopy images showing contracting lattice parameter in both Pt-shell and Au_1−xCu_x-core as a function of Cu at%. HRSTEM images and corresponding FFTs for three compositions of Pt/Au_1−xCu_x nanoparticles are shown, with x = 0.19 for (a–c), x = 0.41 for (d–f) and x = 0.77 for (g–i). (a, d and g) Pt/Au_1−xCu_x nanoparticle with various crystals facets visible. (b, e and h) Atomically resolved region of both core and shell corresponding to the blue squares shown in (a), (d) and (g). (c, f and i) FFTs of the HRSTEM images shown in (b), (e) and (h). The zone axis indicated in the top right corner. All Pt-shells showed a deviation from the literature value for the Pt lattice parameter and demonstrate a lattice strain (when compared to literature value of 0.392 nm for Pt) of +2.42%, +0.36% and −4.03% for (d), (e) and (f), respectively.

Jessi E. S. van der Hoeven