Metals have long stood in the shadow of their semiconductor counterparts when it comes to sparking chemical reactions with solar energy, but chemical engineers have now brought them into the light. Silver nanocubes can use both light and heat to split oxygen molecules, and this new breed of catalyst could help make the production of plastics, antifreeze and other widely-used products more energy efficient and environmentally friendly. “This is the first demonstration of photochemistry on a metal surface using low-intensity visible light,” according to Suljo Linic, an associate professor of chemical engineering at the University of Michigan College of Engineering, in the US. [login type="readmore"] Catalysts are facilitators – they convince other molecules to react while remaining unchanged themselves. Photocatalysts do this by absorbing light and then donating the energy to the reactant molecules. Typically, this energy is ferried to the reactants by charge carriers such as electrons. Metals usually make poor photocatalysts because the electrons cannot gather enough energy if the light is relatively low-intensity, as sunlight is. But Linic and his team showed that a metal surface made of minuscule cubes plays by a different set of rules. LIGHT AND NANOSTRUCTURES [caption id="attachment_8211" align="alignright" width="800"] Silver nanocubes can use both light and heat to split oxygen molecules[/caption] “It’s like a nano-concentrator of light,” said Linic, who led his doctoral students Phillip Christopher, Hongliang Xin and Andiappan Marimuthu on the project. “When you shine light on these nanostructures, they amplify the light intensity, and the electrons start moving back and forth, gaining kinetic energy. “We showed that these energetic electrons can activate chemical transformations in a fundamentally different way than anything else we have seen before,” he added. Light is an electromagnetic wave that oscillates with a particular frequency. The waves can push the electron clouds on metal surfaces, causing them to oscillate as well. The electrons on the team's surface of jumbled silver nanocubes, each 75 nanometers (75 millionths of a millimeter) to a side, naturally synced up with visible frequencies. Those waves amplified the electron oscillations, just as a well-timed push can send a swing higher. In places where the energy built up, usually at the corners and at the edges of cubes, the energetic electrons jumped onto gas molecules that were floating near the silver surface. That gas was a mixture of oxygen and ethylene molecules, and the electrons delivered their energy to the oxygen. When the energy was enough to split the molecules into two atoms, the oxygen could then bind with ethylene and form ethylene oxide, a chemical used in making plastics among other products. By increasing the light’s intensity or the temperature, the team could split oxygen faster and speed up the reaction. The results of their research have been published in the journal Nature Materials. CHEMICALS VS ELECTRONS Chemists have been used heat to shake molecules apart for hundreds of years, but heat is a blunt instrument. It indiscriminately attacks all of the chemical bonds in the reactant molecules, while electrons can target specific bonds. “Electrons can gently push the atoms apart without impacting the rest of the system dramatically,” said Linic. “These electron-driven reactions have not been observed previously with a metal catalyst.” Semiconductors have enjoyed a monopoly on harnessing light to drive reactions with electrons, but although nanostructured metals are new to the field, they have a competitive edge. At higher temperatures, semiconductors are less effective catalysts, yet metals perform better. “The discovery suggests another critical lever in tuning chemical processes,” added Linic. The associate professor of chemical engineering pointed out that most significant applications of fundamental discoveries usually cannot be predicted. “However,” he said, “I can also envision this playing a role in light-induced cleanup of water, various medical applications, and the design of sensors.” If the technique can be scaled up to industrial processing, Linic said the finding’s biggest impact could be a major reduction in chemical waste. Crucial reactions that add oxygen to molecules for making plastics, antifreeze and other goods must often go through intermediate chemicals, generating unwanted byproducts. Chemical engineers may be able to streamline these processes by adding oxygen directly with the help of a nanostructured metal catalyst. The paper is titled, ‘Singular characteristics and unique chemical bond activation mechanisms of photocatalytic reactions on plasmonic nanostructures’. This research work was supported by the US Department of Energy and the National Science Foundation. Courtesy of University of Michigan College of Engineering.