This illustration shows the setup for the experiment at Berkeley Lab's Advanced Light Source that used infrared light (shown in red) and an atomic force microscope (middle and top) to study the local surface chemistry on coated platinum particles (yellow) measuring about 100nm in length. Image: Hebrew University of Jerusalem.Defects and jagged surfaces at the edges of nano-sized platinum and gold nanoparticles are key hot spots for chemical reactivity. This is according to a study conducted with a unique infrared probe by researchers at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the Hebrew University of Jerusalem in Israel. Experiments like this should help researchers to customize the structural properties of catalysts to make them more effective in fostering chemical reactions.
The study, reported in a paper in Nature, is an important step in chronicling how the atomic structure of nanoparticles impacts their function as catalysts in chemical reactions. Catalysts, which play a role in the production of many industrial products, such as fertilizers, fuel and plastics, are materials that can speed up chemical reactions and make them more efficient while remaining unchanged in the process.
Scientists have long known that materials can behave differently at the nanoscale than they do at larger scales, and that customizing their size and shape can enhance their properties for specific uses. In this new study, the researchers pinpointed the areas on single metallic particles around 100nm in size that are most active in chemical reactions.
By combining a broad spectrum of infrared light, produced by Berkeley Lab's Advanced Light Source (ALS), with an atomic force microscope, the researchers were able to reveal the different levels of chemical reactivity at the edges of single platinum and gold nanoparticles. They were also able to compare the reactivity at the edges with that at the nanoparticles’ smooth, flat surfaces.
Taking advantage of a unique capability at ALS, dubbed SINS (synchrotron-radiation-based infrared nanospectroscopy), the researchers explored the detailed chemistry that occurs on the surface of the particles, and achieved a resolution down to 25nm.
"It allows you to see all of this interplay in chemistry," said Michael Martin, a senior staff scientist in charge of infrared beamlines at the ALS. "That's what makes this special." Hans Bechtel, a research scientist at Berkeley Lab who works at the ALS infrared beamlines, added: "You can simultaneously see reactants and the products formed in reactions."
In the experiment, the researchers coated the metallic particles with a layer of reactive molecules and focused the ALS-generated infrared light onto the tiny tip (25nm in diameter) of the atomic force microscope. When coupled with the highly-focused infrared light, the microscope's tip worked like an extremely sensitive antenna to map the surface structure of individual nanoparticles while also revealing their detailed surface chemistry.
"We were able to see the exact fingerprint of molecules on the surface of the particles and validate a well-known hypothesis in the field of catalysis," explained Elad Gross, a faculty member at the Institute of Chemistry and the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem. Gross led the study along with Dean Toste, a faculty scientist in the Chemical Sciences Division at Berkeley Lab and a professor in UC Berkeley's Department of Chemistry.
Knowing the precise level of energy that's needed to trigger chemical reactions (the activation energy) is key to optimizing reactions, and can reduce costs at the industrial scale by conserving energy use. "This technique has the ability to tell you not only where and when a reaction occurred, but also to determine the activation energy for the reaction at different sites," Gross said. "What you have here is a tool that can address fundamental questions in catalysis research. We showed that areas which are highly defective at the atomic level are more active than smooth surfaces."
This characteristic relates to the small size of the particles, Gross noted. "As the particle size is decreased, the structure is less uniform and you have more defects," he said. Smaller particles have a higher surface area per particle than larger particles, which means that more atoms will be located at the edges. Atoms at the edges of the particles have fewer neighbors than those along its smooth surfaces, and fewer neighbors means more freedom to participate in chemistry with other elements.
The studied chemical reactions occur very rapidly – in less than a second – and the ALS technique can take about 20 minutes to scan a single spot on a particle. So the researchers used a layer of chemically-active molecules, which were attached to the surface of the particle, as markers of the catalytic reactivity.
The catalytic reaction in the study was analogous to the reaction that occurs in gasoline-powered vehicles' catalytic converters, which use platinum particles and other materials to convert car exhaust into less-toxic emissions. In future experiments using the SINS technique, the researchers will focus on documenting active chemical processes that use controlled flows of gases or liquids to trigger reactions, and may also use varying pressure and temperature to gauge effects.
"I think this is going to be a very interesting tool for further experiments and analyses that can answer a lot of questions that couldn't be answered before," Gross said. "This tool gives us the capability to get better resolution by three orders of magnitude than some other techniques, which has opened a very wide field for catalysis and surface-chemistry studies."
Future studies could also conceivably combine infrared- and X-ray-based methods at the ALS to gather richer chemical information, the researchers said. There are already plans for a new infrared beamline at the ALS that will increase its capacity and capabilities for infrared chemical studies. This beamline will also help launch infrared-based three dimensional structural studies at the ALS.
This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.