Imaging 3D morphology changes in PtNi nanoparticles under heating using single particle reconstruction

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EMAG - 3D & Tomographic Electron Microscopy
Dr Tom Slater (1), Dr Yichi Wang (3), Dr Gerard Leteba (4), Prof Candace Lang (2), Prof Sarah Haigh (5)
1. Cardiff University
2. Macquarie University
3. Tsinghua University
4. University of Cape Town
5. University of Manchester

Nanoparticles, STEM, 3D Imaging, Tomography, Single Particle Reconstruction, In-Situ, Heating, Catalysis, Oxygen Reduction Reaction, Energy dispersive X-ray spectroscopy.

Abstract text

In this study, we demonstrate the application of a single particle reconstruction (SPR) technique to characterise three dimensional changes to nanoparticle morphology as a function of temperature. PtNi nanoparticles are observed to transform from an etched rhombic dodecahedral morphology to an entirely spherical morphology under heating from 200 °C to 550 °C. The use of an SPR technique to monitor 3D morphological changes of nanoparticles in-situ will enable 3D studies in cases where electron tomography cannot be used (i.e. in closed-cell gas systems).

Understanding the evolution of nanoparticle morphology under reaction conditions is an important step towards understanding variations in activity/selectivity in their use as catalysts1. The development of closed-cell systems for introducing liquids and gases into the transmission electron microscope has led to a number of studies of catalytic nanoparticle morphology changes, giving insight into CO oxidation2,3, CO2 hydrogenation4,5 and many other reactions1. However, the bulky nature of closed-cell holders is restrictive in terms of collecting 3D morphological data through electron tomography (i.e. high tilt-angles are not possible due to collision with the microscope pole-piece).

Here, I will outline how a single particle reconstruction approach can be used as an alternative to electron tomography to collect information on 3D morphological changes when using an in-situ closed cell. Single particle reconstruction is a technique commonly used in structural biology, in which images of many identical particles at different orientations are used to provide a 3D reconstruction6.

In this particular case, single particle reconstruction has been used to characterise the 3D morphological changes of etched rhombic dodecahedral PtNi nanoparticles as they are heated from 200 °C to 550 °C. The PtNi nanoparticles used in this study are of interest as highly active catalysts for the oxygen reduction reaction7. High-angle annular dark field (HAADF) images of 60 nanoparticles were acquired on a Thermo Fisher Titan (S)TEM using Protochips Fusion heating holder at 50 °C intervals (representative nanoparticle image series shown in Figure 1a). Spectrum images were also acquired using energy dispersive X-ray spectroscopy. Individual particles are segmented, their orientation determined through cross-correlation with an initial 3D tomogram, before they are input into a backprojection algorithm using the python API of the EMAN2 software package.

Figure 1. a) HAADF-STEM images of a representative PtNi nanoparticle as it is heated from 200 °C to 550 °C. b) Volume rendering of the 3D reconstruction obtained from 60 PtNi nanoparticles at the heating steps indicated above.

The 3D reconstructions obtained as function of temperature allow observation of changes in morphology and elemental distribution (Figure 1b). In terms of elemental distribution, the nanoparticles start with a Pt-rich core but gradually reach a homogeneous distribution of elements under heating (with Pt enrichment at <111> vertices at certain temperatures). Morphologically, concave surfaces are seen to rapidly change in comparison to convex surfaces, with high-index facets appearing at intermediate temperatures, before proceeding to a spherical morphology.

This study clearly demonstrates the utility of a single particle reconstruction approach to observe changes in the 3D morphology of nanoparticles using bulky in-situ holders. Extension to in-situ holders for the application of liquids, gases and other systems is relatively straightforward and will be pursued in future work. The single particle approach is used here on near-identical PtNi nanoparticles, due to the restriction that the SPR technique requires identical objects. However, we have recently shown a methodology to obtain 3D reconstructions from particles of heterogeneous morphologies8, indicating that this technique could be used on a broad range of nanoparticle samples of non-identical morphology.


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