An international team of researchers led by a group from the University of Manchester in the UK has used a biological technique which won the 2017 Nobel Chemistry Prize to reveal 3D atomic-scale chemistry in metal nanoparticles used as catalysts. It is the first time this technique has been for this kind of research.

The new work, described in an open-access paper in the ACS journal Nano Letters, shows that the metal nanoparticles have a complex star-shaped geometry. The work shows that the edges and corners can have different chemistries which can now be tuned to reduce the cost of batteries and catalytic convertors.

The 2017 Nobel Prize in Chemistry was awarded to Joachim Frank, Richard Henderson and Jacques Dubochet for their role in pioneering the technique of single particle reconstruction. This electron microscopy technique has revealed the structures of a huge number of viruses and proteins but is not usually used for metals.

Now, a team at the University of Manchester, in collaboration with researchers at the University of Oxford and Macquarie University, have built upon the Nobel Prize winning technique to produce three-dimensional elemental maps of metallic nanoparticles consisting of just a few thousand atoms.

The research demonstrates that it is possible to map different elements at the nanometre scale in three dimensions, circumventing damage to the particles being studied.

The properties of nanoparticles are known to critically depend on their local chemistry but characterizing three-dimensional (3D) elemental segregation at the nanometer scale is highly challenging. Scanning transmission electron microscope (STEM) tomographic imaging is one of the few techniques able to measure local chemistry for inorganic nanoparticles but conventional methodologies often fail due to the high electron dose imparted.

Here, we demonstrate realization of a new spectroscopic single particle reconstruction approach built on a method developed by structural biologists. We apply this technique to the imaging of PtNi nanocatalysts and find new evidence of a complex inhomogeneous alloying with a Pt-rich core, a Ni-rich hollow octahedral intermediate shell and a Pt-rich rhombic dodecahedral skeleton framework with less Pt at ⟨100⟩ vertices.

The ability to gain evidence of local surface enrichment that varies with the crystallographic orientation of facets and vertices is expected to provide significant insight toward the development of nanoparticles for sensing, medical imaging, and catalysis.

—Wang et al.

Metal nanoparticles are the primary component in many catalysts, such as those used to convert toxic gases in car exhausts. Their effectiveness is highly dependent on their structure and chemistry, but because of their incredibly small structure, electron microscopes are required in order to provide image them. However, most imaging is limited to 2D projections.

We have been investigating the use of tomography in the electron microscope to map elemental distributions in three dimensions for some time. We usually rotate the particle and take images from all directions, like a CT scan in a hospital, but these particles were damaging too quickly to enable a 3D image to be built up. Biologists use a different approach for 3D imaging and we decided to explore whether this could be used together with spectroscopic techniques to map the different elements inside the nanoparticles.

Like single particle reconstruction, the technique works by imaging many particles and assuming that they are all identical in structure, but arranged at different orientations relative to the electron beam. The images are then fed in to a computer algorithm which outputs a three dimensional reconstruction.

—Professor Sarah Haigh, from the School of Materials, University of Manchester

In the present study the new 3D chemical imaging method has been used to investigate platinum-nickel (Pt-Ni) metal nanoparticles.

Platinum based nanoparticles are one of the most effective and widely used catalytic materials in applications such as fuel cells and batteries. Our new insights about the 3D local chemical distribution could help researchers to design better catalysts that are low-cost and high-efficiency.

—Lead author Yi-Chi Wang

We are aiming to automate our 3D chemical reconstruction workflow in the future. We hope it can provide a fast and reliable method of imaging nanoparticle populations which is urgently needed to speed up optimisation of nanoparticle synthesis for wide ranging applications including biomedical sensing, light emitting diodes, and solar cells.

—Co-author Dr Thomas Slater

Resources

• Yi-Chi Wang, Thomas J. A. Slater, Gerard M. Leteba, Alan M. Roseman, Christopher P. Race, Neil P. Young, Angus I. Kirkland, Candace I. Lang, and Sarah J. Haigh (2019) “Imaging Three-Dimensional Elemental Inhomogeneity in Pt–Ni Nanoparticles Using Spectroscopic Single Particle Reconstruction” Nano Letters doi: 10.1021/acs.nanolett.8b03768