Transmission electron micrograph of typical Pt cubic nanocages.
Transmission electron micrograph of typical Pt cubic nanocages.

A simple, new technique that creates tiny hollow cages of platinum (Pt) with walls just a few atoms thick could reduce the amount of expensive metals needed for catalytic applications, according to US researchers.

Metal catalysts like Pt are widely used to drive a range of industrial and everyday reactions in devices from fuel cells to catalytic converters. But Pt is costly because resources are limited. One strategy is to make a small amount of the metal go much further by creating the largest possible surface area available for reactions to take place. But to achieve this without making catalysts unfeasibly large requires a major reduction in the size of catalyst particles. This strategy is successful at boosting efficiency, but very small nanoparticles can be difficult to handle — their shape can be hard to control and they tend to coalesce into larger particles or detach from their support.

Now, however, researchers from Georgia Institute of Technology and Emory University, University of Wisconsin-Madison, Oak Ridge National Laboratory, Arizona State University, and Xiamen University in China have fabricated nano-sized Pt cages that maximise the surface available for catalysis, while not suffering from the problems that usually plague nanoparticles [Zhang et al., Science 349 (2015) 412].

‘‘By using hollow structures, we can use much larger particle sizes — ∼20 nm — and we really don’t lose any surface area because we can use both the inside and outside of the structure,’’ explains Younan Xia, who led the research. ‘‘This approach creates the highest possible surface area from a given amount of Pt.’’

His lab previously developed a solution-based method for producing core-shell nanostructures where palladium (Pd) nanoparticles are synthesized and then coated with a controlled number of atomic layers of Pt. Now this process has been taken a stage further with an etching step that dissolves the Pd core to leave behind a Pt shell or nanocage (as shown).

While the mechanism is not yet completely clear, the researchers believe the process starts with oxidation of Pd atoms on the surface, which generates vacancies. These vacancies are then filled by Pd atoms diffusing from the core, which are in turn etched away, leaving an empty Pt shell. The cages have walls three to six atomic layers thick — or around 1 nm — and can be either cubic or octahedral in shape, depending on the Pd template, with edges of ∼20 nm.

Octahedral nanocages are particularly attractive for catalytic applications because they possess {111} surfaces (or facets) that are very active for the oxygen reduction reaction (ORR) — the crucial reaction that takes place on the cathode of fuel cells. According to the team’s observations, octahedral Pt nanocages are up to eight times more active than conventional Pt/C catalysts, while cubic nanocages show around five times higher activity. The Pt nanocages are also surprisingly durable, losing only one-third of their ORR catalytic activity over 10,000 operating cycles.

‘‘[The synthesis of] hollow cages and control of facet orientation are attractive features of this approach given the structure sensitivity of the ORR,’’ says Radoslav Adzic of Brookhaven National Laboratory. ‘‘However, competing concepts such as Pt-Ni alloy octahedra and hollow Pt spheres have higher activity or simpler synthesis,’’ he cautions. ‘‘These considerations will determine the impact of this approach.’’

This story was originally published in Nano Today (2015), doi:10.1016/j.nantod.2015.08.001