Researchers have developed a way to build multifunctional nanoscale structures using aluminum nanocrystals. The structures have an aluminum core and are dotted with even smaller metallic islands. The nanocrystals are used as a base for creating size-tunable transition-metal nanoparticle islands that enable localized surface plasmon resonances. According to researchers, aluminum is an effective plasmonic material, and adding smaller catalytic particles from three columns of the periodic table could enhance the structure’s ability to promote chemical reactions driven by photonic energy. Researchers made and characterized aluminum nanostructures decorated with 'islands' of various transition metals (shown here, palladium and ruthenium). The versatile plasmonic allows for customizable surface chemistry and reactivity in one-material nanostructures. Courtesy of Rowan Leary/University of Cambridge. In a two-step method, Rice University researchers first reduced an aluminum precursor to purified aluminum particles between 50 and 150 nm wide. They then suspended the particles in ethylene glycol, added a metal salt precursor and boiled the solution to reduce the salts that eventually nucleated and grew into nano-islands that decorated the surface of the original aluminum nanocrystals. Researchers found that a 2- to 4-nm native aluminum oxide layer separated the aluminum nanocrystal and catalytic nano-islands. “Because a thin layer of aluminum oxide separates the two materials, we can independently tune their properties to suit our needs in future applications,” said researcher Dayne Swearer. In collaboration with scientists at Cambridge University, the Rice team used electron tomography to identify the size and location of more than 500 individual ruthenium nano-islands on a single aluminum nanocrystal. High-resolution and 3D structural analysis using scanning transmission electron microscopy and electron tomography showed that abundant nanoparticle island decoration in the few-nm size range could be achieved, with many islands spaced closely to their neighbors. Nano-islands of ruthenium adhere to an aluminum nanoparticle. Courtesy of Sadegh Yazdi/Rice University. The team used their technique to decorate aluminum nanocrystals with iron, cobalt, nickel, ruthenium rhodium, platinum, palladium and iridium. The islands were as small as 2 nm wide and as large as 15 nm. Researcher Emilie Ringe said that custom-designed devices that couple aluminum and plasmonic islands could make sought-after reactions easier to trigger. According to Swearer, the islands’ small size makes them better at absorbing light than larger nanoparticles and also makes them better at producing hot electrons and injecting them into target molecules for catalysis. “The synthesis could be used to make even more elaborate combinations of metals and semiconductors from the periodic table,” he said. “Each new material combination has the potential to be explored for several applications.” The researchers believe that their general polyol technique could be used to combine multiple materials in a simple, controllable process. The technique could be useful for photocatalysis, surface-enhanced spectroscopy and quantum plasmonics (the study of the quantum properties of light and how they interact with nanoparticles). The research was published in ACS Nano (doi: 10.1021/acsnano.7b04960). This video shows the position of hundreds of ruthenium nano-islands on a single aluminum nanocrystal. Rice University scientists combined aluminum nanoparticles and smaller metal particles to create versatile plasmonic nanostructures. The technique allows for customizable surface chemistry and reactivity in one material. Courtesy of Rice University/University of Cambridge.