A sustainable future for electroanalytical chemistry

Due to its toxicity, mercury use has declined significantly in recent years, with limited applications in the chemical industry (as a catalyst) and in some electrical components. Now, an international team of researchers from Israel, Portugal, and the United States has created a solid form of mercury at room temperature that exhibits unique electrochemical properties with potential for new applications in the fields of electroanalytical chemistry and electrocatalysis.

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Mercury is a unique element, the only metal that exists in liquid form under ambient conditions. Metallic mercury has been widely used in mining, metallurgy, medicine, chemical manufacturing, and in a range of scientific and electrical devices. However, its high vapor pressure and toxicity to humans when inhaling vapors makes metallic mercury potentially deadly.

Nevertheless, the metal finds many applications in medical devices, electrical components, fluorescent lamps and other industrial equipment.

Multi-faceted liquid metal

Mercury is considered a very unusual chemical element with high chemical and biological activity, variability in state (solid, liquid, and gas), and unique properties, such as dissolving other metals to form an amalgam.

The amalgamation process imparts new properties to the resulting amalgam. In particular, the fusion of gold and silver has attracted much interest due to its various applications in metallurgy, chemistry, physics and medicine. Amalgamation also reduces the toxicity of mercury by dramatically lowering its vapor pressure (about 1.6 million times lower than the vapor pressure of the bulk metallic form).

The Challenge of Creating Mercury Nanoparticles

In recent years, metallic nanoparticles have attracted considerable interest from industry and academia as the basis for new technological developments in medicine, electronics, optics, catalysis and many other fields.

Nanoparticles generally exhibit quite different properties from those of bulk materials, and their application has already revolutionized several sciences and industries.

Most of the research at the nanoscale is primarily concerned with metals such as silver and gold, followed by other noble metals and transition metals/rare earths. Despite the attractive characteristics of mercury, its physical properties under ambient conditions hamper the use of the most commonly employed techniques for the fabrication and characterization of nanomaterials.

Recently, a group of scientists from Bar-Ilan University in Israel, led by Professors Aharon Gedanken and Doron Aurbach, together with colleagues from the International Iberian Nanotechnology Laboratory in Portugal and the Argonne National Laboratory in the United States, have created highly stable crystalline mercury nanoparticles that remain solid even under ambient conditions.

Using ultrasound of liquid mercury in water, the scientist succeeded in dispersing the liquid metal into nanodroplets. The mercury-water mixture also contained sheets of reduced graphene oxide (RGO) particles. Following ultrasonication, Professor Gedanken and his colleagues found solid mercury nanoparticles embedded in the RGO layers.

Acoustic cavitation can alter intermolecular bonds

The physicochemical effects of ultrasound arise mainly from the high temperature and high pressure points formed in a liquid medium by the phenomenon of acoustic cavitation (the formation, growth and collapse of bubbles in a liquid).

The physicochemical effects of ultrasound arise mainly from the high temperature and high pressure points formed in a liquid medium by the phenomenon of acoustic cavitation (the formation, growth and collapse of bubbles in a liquid).

In the extreme environment created by the collapse of the bubbles, the temperature and pressure can reach 1,000 to 10,000 K and 107 N m−2respectively, which can break or form intermolecular bonds, thereby improving reaction rates in various chemical processes.

Sonochemistry is also used to synthesize nanoscale crystalline materials by breaking and depositing metals and metal oxides on various substrates.

Ultrasonic energy converts liquid mercury into solid nanoparticles

In the experiments of Professor Gedanken’s team, ultrasound dispersed liquid mercury in the aqueous medium containing GERD sheets. Then, the shock waves and micro-jets created by the collapsing bubbles induced structural reorganization in the nanoscale mercury droplets, forming solid crystals 50-100 nm in size, thanks to a process known as sonocrystallization.

At the same time, the cavitation process also drives the nanoparticles towards the RGO sheets, which have an oxidative surface. This resulting charge transfer interaction between the RGO and the mercury nanocrystals has a stabilizing effect, and the latter become embedded in the RGO sheets.

As Professor Aurbach explains, in fact, scientists have discovered a new composite material, even a new family of composite materials, where metallic mercury is stable in its solid form at ambient conditions.

Solid mercury nanocomposite or sustainable electrocatalysis and electrochemistry

To confirm the crystal structure of the stabilized mercury nanoparticles, the scientist performed a comprehensive analysis of the specimens using various characterization techniques including thermal analysis, X-ray diffraction, and X-ray absorption spectroscopy. , which revealed the stability of mercury nanocrystals at temperatures up to 150°C.

Experimental results also demonstrated a unique electrochemical response of solid mercury nanoparticles. The research team tested the mercury nanoparticle/RGO composite as an electrode material for the hydrogen evolution reaction. They found that the activity of the electrode was comparable to that of other noble metal nanoparticles, such as palladium, ruthenium and gold, loaded onto a carbon substrate.

The electrodes manufactured by Prof. Gedanken’s team contain only a small quantity of solid mercury with a low vapor pressure, therefore very low toxicity, which opens up potential applications in the fields of electroanalytical chemistry and electrocatalysis.

By replacing industry-standard noble metal electrodes, researchers hope to make critical physicochemical processes more sustainable.

Read on: Improving catalytic activities through the observation of nanoparticles.

References and further reading

Harika, VK, et al. (2021) Sustainable existence of solid mercury (Hg) nanoparticles at room temperature and their applications. Chem. Science., 12, 3226-3238. Available at: https://doi.org/10.1039/D0SC06139E

E. Griffiths (2021) Sonic energy transforms liquid mercury into solid nanoparticles [Online] www.chemistryworld.com Available at: https://www.chemistryworld.com/news/sonic-energy-turns-liquid-mercury-into-solid-nanoparticles/4013141.article

Kim, HN; Suslick, KS (2018) The effects of ultrasound on crystals: sonocrystallization and sonofragmentation. Crystals 8, 280. Available at: https://doi.org/10.3390/cryst8070280

Yang, M. et al. (2020) Acoustic cavitation generates molecular mercury(ii) hydroxide, Hg(OH)2from biphasic water/mercury mixtures. Chem. Science., 11, 556-560. Available at: https://doi.org/10.1039/C9SC04743C

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