Space gets smaller with metal nanodroplets like planets

July 14, 2023

(Nanowerk News) Liquid metal, planet-like nanodroplets have been successfully formed with a new technique developed at RMIT University, Australia.

Like our own Planet Earth, the nanodroplets feature an outer ‘crust’, a liquid metallic ‘mantle’ and a solid ‘core’.

A dense intermetallic core is key to achieving a more homogeneous mixture, ‘locking in’ the same amount of solute (i.e., the ‘target’ metal) in each droplet of alloy.

The research team achieved homogeneity through complete dissolution in the molten-metallic medium, which is made possible by the high-temperature molten salt.

This discovery creates new research opportunities in basic liquid metal chemistry as well as applications as diverse as flexible electronics, phase-change materials, catalysts and fuel cells, and silver-based antimicrobials. Planet-like nanodroplets have an outer (oxide) shell, a molten (metallic) mantle, and a suspended, dense (intermetallic) central core. (Image: FLEET)

Molten metal nanodroplets are shaken

Liquid metals have emerged as a promising new frontier of chemical research in recent years, acting as novel reaction interfaces for solvents and catalysts.

They can also act as functional materials offering high conductivity, due to delocalized metallic bonds, and a soft, fluid interior.

With the emergence of catalytic, sensing and nano-electronic applications that rely on achieving large surface areas, the synthesis of liquid metal nanodroplets has become an important focus.

There are many possible combinations when alloying for a particular application, for example dissolving copper (the solute) in liquid gallium (the metal solvent).

Liquid metal nanodroplets are prepared by mechanical agitation using sound waves in a solvent such as ethanol or water.

However, during this ‘sonication’ process, the molten metal alloy tends to ‘de-alloy’, that is, it breaks up into its constituent metals.

This is the result of the previous method of trying to dissolve the metal at relatively low temperatures, close to room temperature. “Just as more sugar dissolves in warm water than cold water, more copper can dissolve in warmer gallium,” said lead author Caiden Parker, PhD candidate at RMIT.

At low temperatures, some of the dissolved metal reforms into larger solid particles before completely dissolving.

The resulting composition has inconsistent and inhomogeneous properties, with the composition of each nanodroplet greatly varying. “In extreme cases, many or even most of the nanodroplets may be essentially devoid of dissolved metals, ending up concentrated in only very few particles,” said corresponding author Dr Torben Daeneke, also at RMIT.

The inhomogeneity and presence of intermetallic compounds poses considerable difficulties for researchers wishing to understand the basic mechanisms at work in liquid metal chemistry.

High temperature and salt form homogeneous, planet-like nanodroplets

“The core is the key!”

In the new study (Advanced Functional Materials, “Synthesis of planet-like liquid metal nanodroplets with promising properties for catalysis”), RMIT researchers solved the dealloying problem by heating the synthesis process significantly (as high as 400°C) to ensure the dissolved metal is completely dissolved and introducing a carefully selected liquid salt suspension fluid.

Sodium acetate was chosen because it remains stable at high temperatures and can be easily removed afterwards.

The resulting nanodroplets display an interesting ‘planet-like’ structure consisting of an outer (oxide) shell, a liquid (metallic) mantle, and a suspended solid (intermetallic) central core.

“We were immediately struck by the resemblance of the nanodroplets to Earth-like planets, with a dense outer shell, mantle of liquid metal and a solid metal core,” said Caiden.

That solid core is the key to the success of the new technique, ‘locking in’ the same amount of solute in each drop of alloy.

“We’re also excited to see our new metallic planet-like nanodroplets everywhere!” continued Caiden.

The system is homogeneously dispersed, with significantly increased output. Transmission Electron Microscope (TEM) analysis confirmed the nuclear structure observed in almost every droplet.

The presence of a dense core also promotes a very attractive use for planet-like nanodroplets in catalytic reactions, ‘accelerating’ chemical reactions.

The studied copper-gallium nanodroplets provide promising results in the electrocatalytic oxidation of ethanol, which can be applied in ethanol fuel cells.

Removal of sodium acetate is important prior to this catalytic reaction, with the salt easily being cleaned in a simple water bath.

What’s next?

The promising new technique unlocks the potential of using high surface area nanodroplets in a variety of future applications, including, but not limited to, electronic or catalytic materials.

The physical scale of nanodroplets (i.e., nano rather than micro) will also aid in basic studies of liquid metal chemistry, including looking into the precise properties of bond formation in liquid metals, solvation abilities, crystallization dynamics and general colloid chemistry that may occur in various liquid metal systems. .

“The planet-like structure is like a tiny miniature laboratory, allowing us to study how liquid metals behave at the atomic level,” said Torben.

While the study proves the feasibility of the new technique using the copper–gallium system, the authors hope for further work to confirm that the technique will work successfully using other combinations of solute and solvent alloying systems, starting with silver, zinc, or bismuth in molten gallium. , tin or indium.

“The main advantage of liquid metal systems is the ability to adapt the alloy to a particular application, depending on the properties of the constituent metals,” said Caiden.

“For example, copper is a great conductor of electricity. When we combine copper with gallium, we not only save significant costs in material consumption, but also pave the way for flexible electronics, like you might see in science fiction movies.”

Potentially, copper can also be utilized for its thermal properties, with the potential application of copper-based nanodroplets in heat dissipation systems.

Applications of nanodroplet catalysis based on copper’s ability to accelerate reactions have been tested in a new study, with an increase in the active site area in addition to savings in material synthesis.

Looking at other metals, silver has previously found applications by virtue of its anti-microbial properties, and once combined with gallium can create a more bioavailable alternative.

“Thus the potential for the application of this new technology is very broad. Any industry that requires nanomaterials can use this system, with the constituent metals varying according to the application,” said Torben.

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