An artificial photocatalyst material made from metal nanoparticles could be used to harvest the sun’s energy more efficiently for use in solar and other applications. Nanostructured materials that demonstrate plasmonic resonances enable intense light focusing. These optical nanoantennas are key to converting free-space light to evanescently confined modes in nanometer-scale volumes below the diffraction limit. A team of researchers from Imperial College London, University of Duisburg-Essen, Rensselaer Polytechnic Institute and Harvard University, led by professor Emilio Cortés from the department of physics at Imperial, have shown the occurrence of light-induced chemical reactions in specific regions on the surface of these nanomaterials. The team identified which regions of the nanomaterial were most suitable for transferring energy to chemical reactions by tracking the locations of very small gold nanoparticles, used as markers, on the surface of nanocatalytic material. These are gold nanoparticles chemically guided inside the hot-spot of a larger gold bow-tie nanoantenna. Courtesy of E Cortés et al, 2017. The team spatially mapped hot-electron-driven reduction chemistry with 15 nm resolution for different plasmonic nanostructures, using a photo-recycling process to modify the terminal group of a self-assembled monolayer on plasmonic silver nanoantennas. According to the team, the resulting localization of reactive regions, determined by hot-carrier transport from high-field regions, could pave the way for improved efficiency in hot-carrier extraction science and nanoscale region-selective surface chemistry. Low efficiency in hot-carrier induced chemical reactions is the biggest unsolved problem that, if resolved, could open the way for large-scale application of artificial photocatalysis. The team showed that by tuning the strength of the electromagnetic (EM) field within the plasmonic antenna and by reducing the mean free path of the carriers with high-local curvature tips, significant enhancement in the efficiency of hot-carrier-induced chemical reactions and/or Schottky barrier photodetection and energy conversion approaches is possible. Through careful design of antennas with sharp tips present only at the structure’s EM hotspots, this method could serve as an efficient, self-guided way of modifying only high EM field regions, while keeping most of the antenna chemically passive (i.e. not reactive). In this way both the most reactive spots (in terms of hot electron transfer) and the most plasmonically active regions (hotspots) of the nanoantennas could be accessed. Attaching molecules to regions with the highest extraction of hot electrons and/or the highest EM fields could further improve the efficiency of photon or electron driven processes such as photocatalysis, (bio)sensing, energy conversion and imaging. This easy, fast and inexpensive strategy could serve as a selective and large-scale method of positioning molecules and nanomaterials in a variety of plasmonic nanoantenna reactive spots or hot spots. “This is a powerful demonstration of how metallic nanostructures, which we have investigated in my group at Imperial for the last ten years, continue to surprise us in their abilities to control light on the nanoscale,” said professor Stefan Maier. “The new finding uncovered by Dr. Cortés and his collaborators in Germany and the U.S. opens up new possibilities for this field in the areas photocatalysis and nanochemistry.” The discovery could lead to better solar panels, as the energy from the sun could be more efficiently harvested. The photocatalyst could also be used drive reactions that break down pollutants into less harmful forms. “This finding opens new opportunities for increasing the efficiency of using and storing sunlight in various technologies,” said Cortés. “By using these materials we can revolutionize our current capabilities for storing and using sunlight with important implications in energy conversion, as well as new uses such as destroying pollutant molecules or gases and water cleaning, among others.” Now that the researchers know which regions of the nanomaterial are responsible for the process of harvesting light and transferring it to chemical reactions, they plan to engineer the nanomaterial to increase the number of regions and make it more efficient. The research was published in Nature Communications (doi:10.1038/ncomms14880).
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