Silver Ions’ Dispersion from Alloys Benefits Optical Sensors, Other Applications

A study led by Rice University and the University of Duisburg-Essen in Germany used hyperspectral dark-field imaging microscopy to reveal the multistep mechanism that causes silver ions to dissipate from gold-silver alloy nanoparticles. Gold-silver alloys are catalysts that can be used to degrade environmental pollutants and, in antibacterial coatings, to kill bacteria on surfaces. In nanoparticle form, the alloys could be useful as optical sensors.

A problem is the tendency of the silver to release from the alloy nanoparticles. The international collaboration supports industry’s ability to fine-tune the nanoparticles for specific uses.

Researchers used gold-silver alloy nanoparticles with an excess of silver in an acidic solution. The hyperspectral dark-field imaging microscopy technique triggered plasmons, ripples of energy that flow across the surface of particles when lit. These plasmons scatter light that changes with the alloy's composition.

The plasmon-on-alloy composition allowed the scientists to record ion-leaching kinetics in real time, said Alexander Al-Zubeidi, lead author of a study describing the work. Silver ions initially leach quickly from nanoparticles, which shrink as a result. As that process continues, the gold lattice (in most cases) releases all the silver over time. About 25% of particles behave differently, though, and the silver leaching process is incomplete.

Chemists quantified the release of silver ions from gold-silver nanoparticle alloys. At top, the change in color as silver (in blue/green) leaches out of a nanoparticle over several hours, leaving gold atoms behind. The bottom images show how much a nanoparticle of silver and gold shrank over four hours as the silver leached away. Courtesy of Rice University.

What the researchers observed suggests that gold could be manipulated over time to stabilize the nanoparticles, Al-Zubeidi said. He believes the silver-release mechanism has been implied from studies of alloy films, though never proven in a quantitative way.

“Usually, silver leaching would last about two hours under our conditions,” he said. “Then in the second stage, the reaction no longer happens on the surface. Instead, as the gold lattice rearranges, the silver ions have to diffuse through this gold-rich lattice to reach the surface, where they can be oxidized. That slows the reaction rate a lot.”

The particles do eventually stabilize, Al-Zubeidi said; the particles passivate, and leaching can no longer happen. So far the researchers have tested only with particles with a silver content of 80% to 90%. They have found that many of the particles will no longer leach silver when they reach a silver content of about 50%.

“That could be an interesting composition for applications like catalysis and electrocatalysis,” Al-Zubeidi said. “We would like to find a sweet spot around 50%, where the particles are stable but still have a lot of their silver-like properties.”

Though the team has yet to look at nanoparticles with silver content outside the 80% to 90% range, the Rice group’s collaboration with Duisburg-Essen chemist Stephan Barcikowski introduced nanoparticle synthesis via laser ablation to the study. “This makes it possible to create alloy nanoparticles with various compositions and free of stabilizing ligands,” said Stephan Link, a chemist at Rice.

"This effort will enable a new approach to generate nanostructured catalysts and new materials with unique electrochemical, optical, and electronic properties,” said Robert Mantz, program manager for electrochemistry at the Army Research Office, an element of the U.S. Army Combat Capabilities Command’s Army Research Laboratory. “The ability to tailor catalysts is important to achieve the goal of reducing soldier-borne weight associated with power storage and generation and enable novel material synthesis.”

The Army Research Office, with a National Defense Science and Engineering Graduate Fellowship; the Robert A. Welch Foundation; the National Science Foundation; and the German Research Foundation supported the study.

The research was published in ACS Nano (www.doi.org/10.1021/acsnano.0c10150).