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Plasmonic Gold Nanoparticles Enhance Charge Transfer for Light-Driven Reactions

Solar has emerged in recent years as the fastest growing renewable energy source in the United States, spurred in part by great improvements in technology that help turn light from the sun into electricity more efficiently.

But there is also a push to use light to do chemistry.

Like electricity, chemicals are vital to everyday life, and it takes a huge amount of energy, generally from non-renewable sources, to convert chemicals into the consumer and industrial products we need, like gases, plastics, paints, pharmaceuticals and so much more.

The chemicals and petrochemicals industries account for about 40% of all industrial energy use and emissions in the U.S., according to the 2022 U.S. Department of Energy Industrial Decarbonization Roadmap.

"A huge amount of energy is spent doing high temperature and high-pressure chemical reactions so that we can get things that we use in our everyday life. So, one of the big picture pursuits of all chemistry right now is trying to figure out a way to do chemistry using light, especially something like sunlight, because that's free," said Christy Landes, professor of chemistry at the University of Illinois Urbana-Champaign.

Landes is part of an Illinois research team and National Science Foundation funded center that has been working in collaboration with researchers at other institutions on this "big picture pursuit." And their efforts have now revealed a different mechanism of charge transfer that is not only much faster than the traditional mechanism but also doubles the total charge transfer efficiency.

In their recently published paper in Science Advances, the Illinois researchers and their collaborators detail the study of this mechanism, which takes advantage of special properties of plasmonic gold particles – 1/1000th the width of a human hair – to transfer charge to a connecting semiconductor of titanium oxide shells.

The researchers identified how the gold nanoparticles transfer charge to a connecting semiconductor and quantified how much charge is transferred using different colors of light.

The work is important, because plasmonic nanoparticles integrated with a semiconductor could improve the efficiency of light-harvesting technology to generate currents or drive chemical reactions.

"Our results reveal how to design better devices that can use these special properties of the metal particles to convert light energy into electrical or chemical energy," said chemistry postdoctoral research associate and co-first author Stephen A. Lee.

Gold nanoparticles absorb a lot of light compared to other particles of the same size and gold is a material that creates collective electronic oscillations when light couples with their surface plasmons. So, the researchers theorized that by exciting the plasmon with light, they would get a boost in charge transfer to the semiconductor material. And their study confirmed their theory.

The researchers report "an overall electron transfer efficiency of 44 ± 3% from gold nanorods to titanium oxide shells when excited on resonance" and half of that "originates from direct interfacial charge transfer mediated specifically by exciting the plasmon."

Stephan Link, professor of chemistry at Illinois and co-lead author on the paper, said that's really what this work is all about, trying to understand the role of the plasmon in charge transfer.

"Is it just a great absorber or does it also help make the charge separated state- And we find out that it does have an additional impact, or boost. That really is what's significant in our work," Link said. "This confirmed our hypothesis that when we excite the plasmon, we're going to get this boost."

At nanoparticle size, gold has a variety of other properties, like tunable color and the ability to drive reactions. The researchers found that the gold particles have much higher charge transfer when they were absorbing a color of light that matched the color of the particles.

"At the plasmon, it absorbs light more strongly, and what we found is that when we do this as a function of color, we see that that the plasmon gives us an extra boost," Link said. "At the plasmon, we're kicking in this extra channel."

Lee compared the channel to a short-cut path to the charge separated state. But gold, like any metal, begins to heat when it absorbs light, and heat can outcompete the short cut.

"So, we're trying to cut out that heating channel that we don't want. Heat can be good for other things, but for what we want to do, we don't want the heat and the plasmon helps us get around the heating," Link said. "The question is, can we intercept before it gets to heating, and the pathways that we described in this paper give one avenue of how to intercept before and get to the charge separated state."

This direct plasmon-induced charge transfer, the researchers explain in their study, is like a long-neglected process known as chemical interface damping of plasmons, a theory put forth in the 1990s.

Landes said this is one part of the study that was so exciting that this old theory was actually very good. Link said more scientists are realizing that this chemical interface damping described in the 90s "as an effect is important in the field of plasmonics and plasmonic photocatalysis.

"This work now for the first time puts complementary techniques together to really understand what chemical interface damping is," said Link, who emphasized that a novel aspect of their study is the variety of different techniques – specifically three different imaging and spectroscopy strategies – that they used to confirm their results.

"This is only possible because we really approached this from many different angles as a team with several different techniques within the NSF funded center led by Christy," Link said.

Their team included researchers at Rice University, Stony Brook University, the University of Wisconsin–Madison, and Stanford University.

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