Original article by Siyuan Zhang, Leo Diehl, Sina Wrede, Bettina V. Lotsch and Christina Scheu. Published in Catalysts 2020, 10, 13.

There is a pressing need to develop renewable solutions for energy conversion and storage. Photocatalysis feeds both birds by one scone, utilizing a semiconductor to harvest solar energy, and co-catalysts to convert the energy into fuels. Such photocatalytic composites are often synthesized in nanometre-size to benefit from large surface areas. Seeing the structure of these “nano-composites” is a fundamental step for rational designs toward higher catalytic activity.

Controlled Heating

In this study, scientists from the Max Planck Institute (MPIE) designed a photocatalytic nanocomposite system with an ultimately thin semiconductor – a single crystal layer of 1 nm thickness. The co-catalysts are similarly tiny, synthesized with controlled heating by a DENSsolutions Wildfire system inside a transmission electron microscope (TEM). Structural evolution of the co-catalysts is captured by in situ TEM observations, based on which a design of co-catalysts with improved photocatalytic activity was demonstrated.

Figure 1. Growth of Ni nanoparticles from the nanocomposite with 10 wt% Ni loading during in situ heating under vacuum. Scale bars in both image sets (a), (b) are 50 nm. (figure from Siyuan Zhang, Max-Planck-Institut für Eisenforschung GmbH, published in https://doi.org/10.3390/catal10010013)

The NiOx co-catalyst was generated from the Ni(OH)2 precursor using a dehydration process. The reaction is irreversible, and the nucleation of the NiOx nanoparticles occurs in a split second.
“The low bulging of the Wildfire chips allows me to keep the atomic resolution imaging with ease and observe the onset of the fast nucleation kinetics.

The latter process of nanoparticle growth is much slower and requires substantial areas for statistics. The fast and reliable temperature ramp up and quench down of the Wildfire chips enables me to monitor the structural evolution over multitudes of areas from global views to atomic resolution imaging.

Moreover, I have run these heating experiments using two generations of Wildfire chips. The sample preparation has become much easier with the new generation, as the droplet of my nanocomposite suspension can be reproducibly dried on the heating area.”

Dr. Spark (Siyuan) Zhang
Max-Planck-Institut für Eisenforschung

The mystery of x

The research is motivated by the fundamental question in materials science, the relationship between structure and properties. The reaction rate is a number to be optimised in photocatalysis. On the other hand, the structure of the studied nanocomposite is multifarious. In addition to capturing the nucleation and growth of NiOx nanoparticles, we reveal the mystery of x in the chemical composition. By repeated heat treatment protocols, nanoparticles during various stages of growth can be “frozen” for microanalysis. By electron energy loss spectroscopy and multi-variate statistical analysis (https://doi.org/10.1093/jmicro/dfx091), a metallic Ni core and an oxidized shell of NiOx is resolved.

Challenging common wisdom

With the resolution power of a modern TEM plus the accurate and stable heating provided by DENSsolutions, we can study the ultimate miniaturized nanocomposite for photocatalysis. The common wisdom to maximize surface areas of catalysts is challenged by our findings, as we exemplify improved activity from core/shell co-catalysts with sufficient spacing between them.

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