Using polyol synthesis methods and inexpensive, abundant aluminum, researchers at Rice University and the University of Cambridge, UK, have developed a flexible, tunable method of fabricating plasmonic heterostructures applicable to a wide range of catalytic processes. These heterostructures consist of an Al nanocrystal core with a thin shell of alumina (aluminum oxide) decorated with islands of transition metals Fe, Co, Ni, Ru, Rh, Pd, Ir or Pt.

By varying the reaction time and the counter-ion attached to the metal precursor, the researchers—including Naomi Halas, Dayne Swearer, Rowan Leary, Ryan Newell, Sadegh Yazdi, Hossein Robatjazi, Yue Zhang, David Renard, Peter Nordlander, and Paul Midgley—can control the composition and size of the catalytic islands as well as their surface coverage. Such flexibility could lead to enhanced, subtle control of the specificity of plasmonic catalytic reactions. 

“Previously, scientists have used polyol synthesis to grow isolated transition metal nanoparticles in solution,” says Emilie Ringe, assistant professor of materials science and nanoengineering and assistant professor of chemistry at Rice University. “We wanted to see if we could use it to nucleate and grow islands of transition metals on the surface of a plasmonic substrate for plasmon-enhanced catalysis,” she says.

The two-step synthesis process involved first creating the Al nanocrystals (AlNCs) by reducing an aluminum hydride precursor, dimethylethylamine alane, by Ti(IV) isopropoxide using a known method.  These AlNCs were then suspended in ethylene glycol (a polyol). A dilute solution of metal salt precursor in ethylene glycol was then added at room temperature. Upon heating to 150°C for 1 hr, the metal salt precursors were reduced to pure metals which decorated the surface of the AlNC. 

Energy dispersive x-ray spectroscopy (EDX) mapping revealed that a 2–4 nm alumina shell had formed around the Al core, insulating the plasmonic Al from the transition metal islands. This separation of the plasmonic metal from the transition metal could eliminate interference in the plasmonic oscillations observed in systems with direct contact between these two metals. High-angle annular dark field scanning transmission electron micrographs (HAADF-STEM) showed that the transition metals had nucleated and grown from the alumina surface, as opposed to homogeneously nucleating in solution and then depositing on the surface.

The resulting “antenna–reactor” heterostructure has a plasmonic Al core that acts as the antenna, interacting with the electric field of light and trapping its energy in oscillations of the conduction band electrons. The islands of catalytic metals decorating the surface act as reactors, adsorbing reactant molecules and releasing product molecules.

Ringe says that such structures have been successfully used in enhancing the selectivity of chemical reactions, as previously published in a study led by collaborator Naomi Halas. Several mechanisms can lead to plasmonic enhancement of catalysis, depending on the reaction and nanostructures; in previous studies the Rice-based scientists have shown that shining light on Pd-decorated Al nanoparticles can lead to highly selective catalysis for selective hydrogenation. Now, they have access to eight different transition metals, each with unique catalytic properties, opening an array of possibilities for new chemical reaction control.

The switch from heat to light as the primary driver for catalysis will require a major redesign of reactor systems—light will not be able to access a packed bed of catalyst in an opaque, metal-walled reactor—but it also opens new possibilities of controlling reaction outcomes. “One of the exciting things is the ability to change reaction selectivity,” Ringe says. “If you illuminate a catalyst rather than heat it, you might gain the ability to push your reaction products in a new direction. I see that as potentially a different mechanism to drive chemistry.”

Developing their flexible, polyol-based synthesis method and demonstrating that it works for eight commonly used catalytic metals is just the beginning. Future work is planned to investigate plasmonic metals other than Al, as well as different catalytic metals, to make these antenna–reactor heterostructures as flexible as possible.

“This work is exciting as it shows that aluminum nanoparticles can be readily decorated with islands of closely coupled catalytic materials” says Sara Skrabalak, James Rudy Professor of Chemistry at Indiana University Bloomington, who was not involved in the current research. “It really opens up the possibility for plasmonic photocatalysis with earth-abundant materials.”

Read the abstract in ACS Nano.