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Researchers Solve a Major Piece of Puzzle of Using Light to Trigger Chemical Reactions

How is it possible to catalyze chemical reactions by light, using the example of photosynthesis in nature? This process is still not understood well.

However, scientists from Johannes Gutenberg University Mainz (JGU) in Germany and Rice University in Houston, USA, have now unraveled a major piece of the mystery. The study outcomes have been reported recently in Science Advances.

Bushes, trees, and other plants are very efficient in turning carbon dioxide and water into oxygen and glucose, a kind of sugar, with the help of photosynthesis. If the underlying basic physical mechanisms are understood and if they are tapped for other common applications, the advantages for mankind could be vast. For instance, the energy of sunlight could be used to produce hydrogen from water as a fuel for automobiles. The method of employing light-driven processes such as those taking place in photosynthesis in chemical reactions is known as photocatalysis.

Plasmons: Electrons Oscillating in Synchrony

Researchers generally use metallic nanoparticles to acquire and utilize light for chemical processes. The exposure of nanoparticles to light in photocatalysis leads to the formation of what are called plasmons. These plasmons are collective oscillations of free electrons in the material.

Plasmons act like antennas for visible light.

Professor Carsten Sönnichsen, Mainz University

However, the physical processes happening in photocatalysis involving such nano-antennas is not yet understood thoroughly. The research groups at JGU and Rice University have now managed to cast some light on this mystery. This process has been more widely studied by the graduate student Benjamin Förster and his supervisor Carsten Sönnichsen.

Modifying Plasmon Resonances

Förster mainly focused on finding out how illuminated plasmons reflect light and at what intensity. His method utilized two very specific thiol isomers, molecules whose structures are organized as a cage of carbon atoms. There are two boron atoms within the cage-like structure of the molecules. By changing the positions of the boron atoms in the two isomers, the scientists were able to modify the dipole moments, or to put it in another way, the spatial charge separation over the cages.

This resulted in a remarkable finding: When they applied the two varieties of cages to the surface of metal nanoparticles and excited plasmons using light, the plasmons reflected various quantities of light based on the cage, which was currently on the surface. In a nutshell, the chemical nature of the molecules placed on the surface of gold nanoparticles had an influence on the local resonance of the plasmons since the molecules also change the electronic structure of the gold nanoparticles.

Teamwork Crucial for Results

Cooperation was important in the project. “We would never have been able to achieve our results single-handedly,” stated Sönnichsen. Benjamin Förster spent a year funded by the Graduate School of Excellence Materials Science in Mainz (MAINZ) investigating at Rice University in Houston with Professor Stephan Link, who has been a visiting professor at MAINZ since 2014.

While the funding of the MAINZ Graduate School offered by the German federal and state governments’ Excellence Initiative will be ending in October 2019, Mainz University will—in special situations—continue to offer postgraduates with financial support for this sort of long-term stays abroad. This will be organized under the sponsorship of the Max Planck Graduate Center (MPGC) and together with the state of Rhineland-Palatinate.

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