Real-time Imaging of Nanoparticle Restructuring

Oxidation phenomena have long been known, even in the case of iron nanoparticles, which can form hollow oxide shells. However, observing such processes at reaction conditions, in real time, at atomistic resolutions and in three dimensions has so far remained a scientist’s dream.

Now, researchers from Temple University and Argonne National Lab have discovered an imaging method based on combined X-ray scattering and computer simulations that turns the dream into reality.

We often admire the beautiful turquoise tops found on old buildings, though their constructers had originally installed copper coloured ones. Exposed to rain and air, the copper has in the meantime oxidised and transformed into the turquoise copper oxide.

Oxidation phenomena have been known for a long time, and nowadays they are widely utilised in various technologies, such as for the production of clean fuels, catalysis or electrochemical energy storage.

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Since the 19^th century we know that on an atomic level the metal atoms rearrange during oxidation.  Nowadays, we usually study the oxidation phenomena by characterising the material after the reaction has taken place.

Advances in electron microscopy has also made it possible to monitor the process in-situ, which means in real time, while it is happening. Yet, with this technique only two dimensional (2D) projections of the oxidation process under vacuum conditions can be obtained.

Novel 3D method detects formation of hollow cores

Now, researchers from Temple University and Argonne National Lab have used time-resolved X-ray scattering techniques in combination with computational simulations to monitor the oxidation process of iron (Fe) to Fe oxide nanoparticles for the first time in three dimensions (3D), with great resolution and at ambient conditions [1].

With this, the scientists around Yugang Sun have invented a method to observe material restructuring processes in real time. The finding, recently published in Science, opens up for further research of outstanding fundamental questions in chemistry and materials science.  

Using the novel method, Sun and colleagues could observe that the surface layer first oxidised at ambient conditions forming a protective Fe oxide shell. Further oxidation then introduced more and more voids in the nanoparticle, which eventually fused together until a complete hollow interior was formed.

The researchers identified that a so-called Kirkendall effect caused the formation of the hollow particle. The effect describes a transport (also known as diffusion) process where metal travels faster out of the particle through the outer oxide layer than the oxygen travels inwards. Overall, material is transported outwards, leaving the particle with a hollow core.

Combined experiment and theory delivered unprecedented imaging resolution

Although hollow Fe oxide nanoparticles are already in use as electrodes for battery applications, this is the first time that the formation process has been observed in such detail.

Doris Cadavid and Andreu Cabot from the Catalonia Institute for Energy Research, who have not been involved in the study, comment in a Perspectives piece in the same issue of Science: “The real-time monitoring at the atomic level of such reactive diffusion processes is extremely challenging” [2].

The resolution of the novel technique was smaller than one nanometre, which already is several thousands of times thinner than a sheet of paper. The researchers achieved this by a combination of experimental and theoretical techniques.

They used small-angle X-ray scattering at the Advanced Photon Source, a synchrotron-radiation facility at Argonne, to characterise the void structures, and wide-angle X-ray scattering to obtain information about the crystallinity of the nanoparticles. With complementing computer simulations, the scientists were able to track movement atom by atom.

"This truly exemplifies how the sum can be greater than the parts -- how theory and imaging together give us information that is better than what can be obtained when these methods are used independently", says computational scientist Subramanian Sankaranarayanan, who is one of the principal investigators of the study [3].

Paving the way for exotic nanoscale materials

The novel method can be used to obtain a deeper understanding of diffusion processes that lead to a restructuring of a material, such as the Kirkendall effect. “If well understood, [the Kirkendall effect] can be used to design exotic materials at the nanoscale," says Sankaranarayanan [3]. For example, by understanding and manipulating the restructuring processes, improved catalysts or intermetallic compounds, such as carbon steel, could be created. Badri Narayanan, co-author of the study adds [3]: "Without understanding these processes as they naturally occur, you can never hope to control them to produce new materials with exceptional functionality".

References:

1. "Quantitative 3D evolution of colloidal nanoparticle oxidation in solution" - Y. Sun et al, Science, 2017, DOI:10.1126/science.aaf6792
2. "Oxidation at the atomic scale" - D. Cadavid and A. Cabot, Science, 2017, DOI:10.1126/science.aan0979
3. Revealing the mystery behind the formation of hollowed nanoparticles during metal oxidation
4. "New study reveals the mystery behind the formation of hollowed nanoparticles during metal oxidation" - Argonne National Laboratory
5. Image Credit: Shutterstock.com/KaterynaKon

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