Opening a wide window on the nano-world of surface catalysis

June 6, 2014 by Steven Powell
Opening a wide window on the nano-world of surface catalysis
Two different etching methods produced two different kinds of nanoparticles: nanorice (top) and nanodumbbells. In both, the original flat surface of the nanocuboid was replaced by a curved surface with more exposed, catalytically active atoms. Credit: American Chemical Society

( —Surface catalysts are notoriously difficult to study mechanistically, but scientists at the University of South Carolina and Rice University have shown how to get real-time reaction information from Ag nanocatalysts that have long frustrated attempts to describe their kinetic behavior in detail.

The key to the team's success was bridging a size gap that had represented a wide chasm to researchers in the past. To be effective as nanocatalysts, noble metals such as Au, Pt, Pd and Ag typically must be smaller than 5 nm, says Hui Wang, an assistant professor of chemistry and biochemistry at South Carolina who led the team in collaboration with Peter Nordlander of Rice University.

Unfortunately, 5 nm is below the size threshold at which plasmon resonance can be effectively harnessed. Plasmon resonance is a phenomenon giving rise to a dramatic enhancement of impinging electromagnetic signals, which is the basis of such as enhanced Raman spectroscopy (SERS).

The ability to utilize the analytical power of plasmon resonance in a nanomaterial requires larger nanoparticles, "at least tens of nanometers in diameter," says Wang. The incompatibility of the two size regimes had long precluded the use of a range of spectral techniques based on plasmon resonance—SERS is just one—on noble metal nanocatalysts under 5 nm.

But as they just reported in Nano Letters, Wang and his team managed to combine the best of both size worlds.

Etching can be halted at different stages to produce a range of sizes and shapes of nanoparticles. Nanocuboids (top) and nanodumbbells at two different etching stages (middle and bottom) are shown here. Credit: American Chemical Society

Starting with cuboidal nanoparticles about 50 nm wide and 120 nm long, they chemically etched flat surfaces in a way that generated curved surfaces, creating nanoparticles that successfully catalyzed a model surface hydrogenation reaction. According to the team, the catalysis is the result of replacing low-energy atoms on the flat surface with exposed atoms after etching.

"If you have a , the coordination number of every single surface atom is either eight or nine," says Wang of their nanoparticles, which had a surface of pure Ag before etching. "But if you have some atomic steps on a surface, the will decrease. These exposed atoms are more active."

The stepped surface of the etched nanomaterial thus mimics the environment of a sub-5-nm nanoparticle: more exposed, active surface atoms can participate in catalysis.

And the catalysis is on a nanoparticle with plasmonic activity, which the researchers showed can be "tuned" by varying the shape and size of the nanoparticles. The team demonstrated the ability to convert cuboids (something like a short rod but with square rather than round sides) into what they termed "nanorice" and "nanodumbbells" through two different kinds of chemical etching. The two shapes had distinct plasmonic properties that could be varied by stopping the etching at different stages to create different sizes and shapes of nanoscale rice and dumbbells.

The spectral properties of the different nanoparticles are size- and shape-dependent and can be tuned by varying etching times. Credit: American Chemical Society

That plasmonic activity can be harnessed for SERS and other analytical techniques to study catalytic reactions in great detail as they occur.

"Raman spectroscopy is extremely powerful, with information about molecular fingerprints—you can see the structures, you can tell how the molecules are oriented on the surface," Wang says. "If you want to use GC, HPLC, or mass spec, you have to damage a sample, but here you can actually monitor the reaction in real time.

"And there is much more information with this approach. For example, we identified the intermediate along the reaction pathway. With those other approaches, it's really hard to do that."

Explore further: Silver nanoparticles on graphene oxide support

More information: Paper:

Related Stories

Silver nanoparticles on graphene oxide support

March 3, 2014

Silver (Ag) has a high catalytic activity towards many organic and inorganic transformations such as NOx reduction and catalytic oxidation of CO to CO2. In practical applications, catalysts like Ag are affixed to a substrate, ...

Hot electrons do the impossible in catalytic chemistry

December 17, 2012

(—From petroleum refining to food processing, the vast majority of commercial chemical applications involve catalysts to control the rate of chemical reactions. Anything that can increase the efficiency of catalysts ...

Silver nanoparticles trap mercury

February 16, 2012

( -- Anyone who thinks amalgams are limited to tooth fillings is missing something: Amalgams, which are alloys of mercury and other metals, have been used for over 2500 years in the production of jewelry and for ...

Recommended for you

Going nano in the fight against cancer

August 17, 2017

Imagine being able to see the signs of cancer decades before we can now. URI Chemical Engineering Assistant Professor Daniel Roxbury and researchers from Memorial Sloan Kettering Cancer Center have invented a technique that ...

Multicolor MRIs could aid disease detection

August 16, 2017

Researchers at Case Western Reserve University School of Medicine have developed a method that could make magnetic resonance imaging—MRI—multicolor. Current MRI techniques rely on a single contrast agent injected into ...

Turning pollen into a low-cost fertilizer

August 16, 2017

As the world population continues to balloon, agricultural experts puzzle over how farms will produce enough food to keep up with demand. One tactic involves boosting crop yields. Toward that end, scientists have developed ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.