Solvated Electrons and Hydroxyl Radicals at the Plasma-Liquid Interface

TL;DR

This study investigates the distribution and interaction of solvated electrons and hydroxyl radicals at the plasma-liquid interface, revealing their penetration depths, concentrations, and reaction mechanisms under different bias voltages.

Contribution

It provides the first direct measurement of solvated electron penetration depth and concentration at the plasma-liquid interface using lock-in amplification absorbance, elucidating their interactions with hydroxyl radicals.

Findings

Penetration depth of solvated electrons is about 10 nm.

Peak concentration of solvated electrons reaches 1 mM near the surface.

Hydroxyl radicals mainly react with solvated electrons at the interface.

Abstract

While hydroxyl radicals ($\cdot$OH) play an important role as potent oxidizing agents in various plasma applications, their high reactivity confines them to a thin layer at the plasma-liquid interface, posing challenges in comprehending the intricate generation and transport processes. Similarly, solvated electrons ($\mathrm{e_{aq}}$), highly reactive reducing agents, are expected to exhibit distribution beneath the liquid surface and interact with $\cdot$OH in the thin layer. In this study, we have determined the penetration depth and concentration of ($\mathrm{e_{aq}}$) at the interface between an atmospheric argon plasma plume and an electrolyte anode via a lock-in amplification absorbance measurement. With bias voltages from 1000 to 2500 V, the penetration depth remains approximately 10 nm, and the peak concentration near the surface reaches 1 mM. Diffusion is the primary mechanism for $\cdot$OH generation in the electrolyte, with most $\cdot$OH reacting with ($\mathrm{e_{aq}}$) at the interface, thus influencing the ($\mathrm{e_{aq}}$) distribution. In contrast, the electrolyte cathode significantly boosts $\cdot$OH generation, leading to rapid recombination into hydrogen peroxide.