Counting individual electron charges could improve nanoparticle catalysts

A single platinum nanoparticle observed by electron holography. (Courtesy: Murakami Lab, Kyushu University)

How many electrical charges does a platinum nanoparticle have? Thanks to an improved high-precision electron holography technique, it is now possible to answer this question by counting the charges directly, down to the level of a single electron. The technique, developed by researchers at Kyushu University and Hitachi Ltd in Japan, could help scientists create more efficient catalysts.

Removing just one or two negative charges from a nanoparticle can significantly change its behaviour as a catalyst. For this reason, determining the charge state of individual nanoparticles on a metal oxide surface is an important task for catalyst engineering, explains team leader Yasukazu Murakami, a quantum materials scientist at Kyushu. The problem is that current techniques to do this, such as X-ray photoemission spectroscopy, only provide charge information averaged over many nanoparticles.

Electron holography

In the new work, the researchers employed electron holography (a type of transmission electron microscopy) to directly identify the electrostatic potential created by nanoparticles of platinum on a surface of titanium oxide – a combination of materials frequently used as a catalyst to speed up chemical reactions. In electron holography, an electron interacting with electric and magnetic fields produces a phase shift in the electron’s wavefunction that can then be identified by comparing it to a reference electron that hasn’t interacted with a field.

By measuring the fields around the platinum nanoparticles, Murakami and colleagues determined the number of “extra” or “missing” electrons associated with them. Their measurements showed that a nanoparticle could gain or lose anywhere between one and six electrons.

The researchers say that the mechanism behind platinum charging involves a difference in the work functions (the energy required to completely withdraw an electron from a metal surface) of platinum and titanium dioxide (TiO2). This difference depends on the orientation of the nanoparticles on the TiO2 and the distortion of the crystal lattice.

Reducing mechanical and electrical noise

A central element in the researchers’ achievements was a series of improvements made to a 1.2-MV atomic-resolution holography microscope developed and operated by Hitachi. This instrument reduces mechanical and electrical noise and then processes the data to further tease out the signal from the noise, Murakami explains.

“High-precision electron holography could be applied to cutting-edge studies in condensed-matter physics, inorganic chemistry, including catalysis, spintronic/semiconductor devices, new types of batteries and other subjects in which a comprehensive electromagnetic field analysis is essential,” he tells Physics World.

In this study, which is detailed in Science, the researchers measured the charge on single nanoparticles in a vacuum. However, in the future they hope to repeat their experiments in a gaseous environment. “Such studies would reflect the conditions in which working catalysts are employed,” Murakami says.

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