Why Some Metal Ions Spontaneously Form Nanoparticles in Water Microdroplets? Disentangling the Contributions of Air-Water Interface and Bulk Redox Chemistry

TL;DR

This study investigates the spontaneous formation of metal nanoparticles in various solvents, revealing that redox chemistry, not the air-water interface, drives nanoparticle formation in microdroplets and bulk solutions.

Contribution

It demonstrates that metal nanoparticle formation occurs in bulk solutions and is driven by redox reactions, challenging the idea that the air-water interface is the primary factor.

Findings

Metal nanoparticles form spontaneously in bulk solutions containing metal salts.

Redox reactions involving water or alcohols accompany nanoparticle formation.

The process is not limited to water or gold ions and occurs in solvents with hydroxyl groups.

Abstract

Water microdroplets containing 100 micromolar HAuCl4 have been shown to reduce gold ions into gold nanoparticles spontaneously. It has been suggested that this chemical transformation is driven by ultrahigh electric fields at the air-water interface, albeit without mechanistic insight. We investigated the fate of several metallic salts in water, methanol, ethanol, and acetonitrile in bulk and microdroplets. This revealed that when HAuCl4 (or PtCl4) is added to bulk water (or methanol or ethanol), metal NPs appear spontaneously. Over time, the nanoparticles grow in bulk, as evidenced by the solution's changing colors. If the same bulk solution is sprayed pneumatically and collected, the NP size has no significant enhancement. Interestingly, the reduction of metal ions is accompanied by the oxidation of water (or alcohols); however, these redox reactions are minimal in acetonitrile. We establish that the spontaneous reduction of metal ions is (i) not limited to water or gold ions, (ii) not driven by the air-water interface of microdroplets, and (iii) appears to be a general phenomenon for solvents containing hydroxyl groups. These results advance our understanding of liquids in general and should be relevant in soil chemistry, biogeochemistry, electrochemistry, and green chemistry.