Researchers at Rice University and the University of Cambridge made and characterized aluminum nanostructures decorated with 'islands' of various transition metals (above, palladium and ruthenium). Image: Rowan Leary/University of Cambridge.Individual nanoscale nuggets of gold, copper, aluminum, silver and other metals that can capture light's energy and put it to work are being employed by Rice University scientists to build multifunctional nanoscale structures.
The structures have an aluminum core and are dotted with even smaller metallic islands. This allows them to sustain localized surface plasmon resonances, collective oscillations of electrons inside the nanostructure that activate when light hits the particle. These nanoscale oscillations in electron density can power chemical reactions, turning the structures into catalysts.
The technique developed in the labs of Rice materials scientists Emilie Ringe and Naomi Halas uses aluminum nanocrystals as a base for size-tunable transition metal islands that promote localized surface plasmon resonances. Aluminum is an effective plasmonic material, but adding smaller catalytic particles from three columns of the periodic table enhances its ability to promote chemical reactions driven by light's energy, as shown in a previous collaboration between the Halas and Ringe groups.
The technique allows for customizable surface chemistry and reactivity in one material, the researchers said. It 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 is reported in a paper in ACS Nano.
The researchers have developed a general technique for combining multiple materials in a simple, controllable process. Rice graduate student and lead author Dayne Swearer and his colleagues used a two-step synthetic method that begins with the reduction of an aluminum precursor to produce purified aluminum particles between 50nm and 150nm wide. They then suspend these particles in ethylene glycol, add a metal salt precursor and boil the solution to reduce the salts, which eventually nucleate and grow into nano-islands decorating the surface of the original aluminum nanocrystals.
Using an electron microscope, the researchers found that a 2–4nm native aluminum oxide layer separated the aluminum nanocrystal and catalytic nano-islands. Additionally, in collaboration with Rowan Leary and Paul Midgley, material scientists at Cambridge University in the UK, the team was able to use electron tomography to identify the size and location of more than 500 individual ruthenium nano-islands on a single aluminum nanocrystal.
"The naturally occurring nanoscale geometry of these new materials is really exciting," Swearer said. "Because a thin layer of aluminum oxide separates the two materials, we can independently tune their properties to suit our needs in future applications."
The lab used the method to decorate aluminum nanocrystals with iron, cobalt, nickel, ruthenium rhodium, platinum, palladium and iridium. The islands were as small as 2nm wide and as large as 15nm. Custom-designed devices that couple aluminum and plasmonic islands will make sought-after reactions easier to trigger, Ringe said.
In 2016, the team showed that aluminum nanocrystals decorated with palladium islands, made using a different method, could promote selective hydrogenations when exposed to light that were not possible when the material was simply heated in the dark. "We hope that with this new, expansive library of similar nanomaterials, many new types of previously inaccessible chemical reactions will become possible," Swearer said.
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," said Swearer. "Each new material combination has the potential to be explored for several applications."
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.