Editorial Feature

Solid Mercury Nanoparticle Synthesis Suggests Sustainable Future for Electroanalytical Chemistry

Due to its toxicity, the use of mercury significantly declined 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 USA created a solid form of mercury at room temperature that exhibits unique electrochemical properties with a potential for novel applications in the fields of electroanalytical chemistry and electrocatalysis.

Mercury Nanoparticle

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Mercury is a unique element, the only metal that exists as a liquid under ambient conditions. Metallic mercury has been used widely in mining, metallurgy, medicine, chemical manufacturing, and in a range of scientific and electrical apparatus. However, its high vapor pressure and toxicity to humans upon vapor inhalation make metallic mercury potentially lethal.

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

Liquid Metal with Many Faces

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

The amalgamation process imparts novel properties to the resulting amalgam. In particular, the amalgamation of gold and silver has gained much interest due to its various applications in metallurgy, chemistry, physics, and medicine. The amalgamation also reduces the toxicity of mercury by significantly decreasing its vapor pressure (approximately 1.6 million times lower than the vapor pressure of the bulk metallic form).

The Challenge of Creating Mercury Nanoparticles

In recent years, metal nanoparticles have gained considerable interest from the industry and academia as a basis for novel technological developments in medicine, electronics, optics, catalysis, and many other areas.

Nanoparticles usually exhibit quite different properties compared to bulk materials, and their application has already revolutionized several sciences and industries.

The bulk of the nanoscale research relates mainly to metals such as silver and gold, followed by other noble metals and transition/rare earth metals. Despite mercury's attractive characteristics, its physical properties under ambient conditions hinder 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 USA, created highly stable crystalline mercury nanoparticles that remain solid even under ambient conditions.

By ultrasonication 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. As a result of the ultrasonication, Prof. Gedanken and his colleagues found solid mercury nanoparticles embedded into the RGO layers.

Acoustic Cavitation Can Alter Intermolecular Bonds

The physicochemical effects of ultrasound derive primarily from high-temperature and high-pressure spots formed within a liquid media through the phenomenon of acoustic cavitation (the formation, growth, and collapse of bubbles in a liquid).

The physicochemical effects of ultrasound derive primarily from high-temperature and high-pressure spots formed within a liquid media through the phenomenon of acoustic cavitation (the formation, growth, and collapse of bubbles in a liquid).

In the extreme environment created by the collapsing bubbles, the temperature and pressure can reach 1000–10,000 K and 107 N m−2, respectively, which can break or form intermolecular bonds, thus improving the reaction rates in various chemical processes.

Sonochemistry is also employed to synthesize nanosized crystalline materials by breaking and depositing metals and metal oxides onto various substrates.

Ultrasound Energy Converts Liquid Mercury into Solid Nanoparticles

In Prof. Gedanken's team experiments, the ultrasonication dispersed the liquid mercury within the aqueous medium containing RGO sheets. Next, the shockwaves and micro-jets created by the collapsing bubbles induced structural reorganization in the nanoscale mercury droplets, forming solid crystals, 50-100 nm in size, through 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 within the RGO sheets.

As Prof. Aurbach explains, in effect, the scientists discovered a new composite material, or 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 crystalline 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, that revealed stability of the mercury nanocrystals at temperatures up to 150 °C.

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

The electrodes made by Prof. Gedanken's team only contain a small amount of solid mercury exhibiting low vapor pressure, hence very low toxicity, which opens potential applications in the fields of electroanalytical chemistry and electrocatalysis.

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

Continue reading: Improving Catalytic Activities Through Nanoparticle Observation.

References and Further Reading

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

E. Griffiths (2021) Sonic energy turns 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, H.N.; Suslick, K.S. (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)2, from biphasic water/mercury mixtures. Chem. Sci., 11, 556-560. Available at: https://doi.org/10.1039/C9SC04743C

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Cvetelin Vasilev

Written by

Cvetelin Vasilev

Cvetelin Vasilev has a degree and a doctorate in Physics and is pursuing a career as a biophysicist at the University of Sheffield. With more than 20 years of experience as a research scientist, he is an expert in the application of advanced microscopy and spectroscopy techniques to better understand the organization of “soft” complex systems. Cvetelin has more than 40 publications in peer-reviewed journals (h-index of 17) in the field of polymer science, biophysics, nanofabrication and nanobiophotonics.

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