New guidelines for designing nanoparticles could help researchers increase the efficiency of plasmonic devices for solar energy conversion. These design principles emerged from a computational study exploring nanoparticles with a bimetallic core-shell design by Imperial College London researchers. This work was recently published in Computational Materials.

In a conducting nanoparticle used in solar photocatalysis, the absorption of sunlight can induce the collective motion of electrons known as a plasmon. The decay of plasmons generates highly excited electrons and holes known as hot carriers that can be harnessed for difficult chemical reactions, like those needed to split water. There is especially a lot of interest in developing solar-powered nanoplasmonic hot-carrier devices that can produce hydrogen fuel, among other applications.

Plasmonic devices have been developed based on single-metal nanoparticles, but their energy conversion efficiencies are low. Research suggests that replacing single-metal nanoparticles with core-shell nanoparticles, in which a metallic core is surrounded by a shell of a different metal, can yield major improvements. Core-shell nanoparticles are commonly used in experiments but little is known about their hot-carrier generation properties and how the properties vary with metal choice.

To determine the optimal combination of metals for water splitting applications, a research team led by Johannes Lischner of Imperial College computationally studied 100 core-shell nanoparticles in an aqueous environment under sunlight. The nanoparticles were bimetallic combinations of 10 elements chosen for their chemical diversity, experimental relevance, and availability of bulk data (Ag, Al, Au, Cu, K, Li, Mg, Na, Sn, and Zn).

Using a recently developed approach based on effective mass theory, the researchers calculated the number of hot electrons and holes generated by each core-shell combination that could trigger water-splitting reactions. The results show that a bimetallic core-shell nanoparticle often has better photocatalytic properties than a simple nanoparticle composed of either component metal.

The data also show that core-shell nanoparticles with alkali or alkali-earth metals tend to generate more hot electrons than transition metal systems. In contrast, core-shell nanoparticles with transition metals tend to generate more hot holes. Combining the two in a single core-shell nanoparticle yields high numbers of both types of carriers, which is key to achieving water splitting. Furthermore, the research team found the outcome to be most promising when the transition metal is in the shell.

According to the researchers, the mechanism behind this result can be explained by a simple argument. “[T]here is an important analogy between core-shell nanoparticles and well-studied photocatalytic semiconductors, such as TiO₂,” Lischner says. “TiO₂ can induce chemical reactions when two energy levels, known as the conduction band minimum and the valence band maximum, have the required values. Similarly, core-shell nanoparticles can be characterized by two energy levels, the work functions of the core and the shell,” he says.

When the work function of the shell is greater than the work function of the core, the electrons confined in the shell behave like a two-dimensional electron gas and so most of the hot carriers are generated in the shell. When the work function of the core is greater than the shell, most of the hot carriers are generated in the core. Hot carriers in the shell are more easily extracted than hot carriers in the core; therefore, the most effective combination is a low work function core and a high work function shell, as in the case of a transition metal shell and an alkali metal core.

The researchers also explored how nanoparticle size and shell thickness influence performance, finding that hot-carrier properties are highly sensitive to both. The optimal radius peaks around 5 nm for core-shell nanoparticles and a thin shell optimizes a particle’s absorption capacity.

Alejandro Manjavacas, an expert in nanophotonics from the University of New Mexico who was not associated with this research, says that the detailed performance analysis of such a large number of nanoparticles with different materials and geometries will be very useful for the community, especially for helping experimentalists choose the optimal nanostructures for their experiments. As a result of these design tips, he says, “I am confident this work will help to develop new applications in plasmon-enabled photocatalysis.”

Read the article in Computational Materials.