Solvated Electron Chemistry at a Plasma-liquid Interface

Low-temperature, atmospheric pressure plasmas provide a convenient means for delivering a variety of reactive chemical species to aqueous solutions.Researchers have thoroughly studied many of the neutral radicals commonly produced by air plasmas, such as atomic oxygen (O), atomic hydrogen (H), hydroxyl radicals (OH), hydroperoxyl radicals (HO₂), atomic nitrogen (N), and nitrous oxides (NO_x).By and large, these neutral radicals ultimately yield the stable products hydrogen peroxide (H₂O₂), nitrous acid (HNO₂), and nitric acid (HNO₃) in the bulk liquid.Recently, more attention has been given to the free electrons in the plasma, and it has been hypothesized that these electrons are responsible a number of electrolytic reduction reactions in aqueous solution, such as the reduction of heavy metal ions to form metallic nanoparticles.If these reduction reactions are indeed driven by free electrons from the plasma, the electrons likely become solvated into solution before initiating any reduction reaction.

In this work, it is experimentally shown that free electrons from the plasma become solvated before initiating electrolytic reactions just beneath the plasma-liquid interface.Solvated electrons are known to absorb red light, therefore the experimental method uses optical absorption spectroscopy in a novel reflection geometry to directly probe the interface.The solvated electrons have a lifetime ~ 1 μs, making them difficult to detect, so a lock-in amplification technique is used to attain a sensitivity of 1 part per 10⁶.

The experimental results indicate that the solvated electrons penetrate 2.5 nm into solution before reacting away.Bulk measurements of OH^-_(aq) and H_2(g) indicate that the electrons primarily react away via 2^nd order recombination, 2e^–_(aq) + 2H₂O → 2OH^–_(aq) + H_2(g)­.Introducing known electron scavengers, such as H₂O₂, acid (H⁺), nitrite (NO₂^-), and nitrate (NO₃^-), into the solution quenches the signal in a manner consistent with reaction kinetics previously measured in radiolysis experiments.Additionally, it is found that introducing O₂ gas into the plasma phase rapidly quenches the signal due to electron attachment, e^– + O_2(g) → O₂^–.Furthermore, operating the plasma in air is found to produce NO₂^-_(aq) and NO₃^-_(aq) via the dissolution of NO_x gas into the solution, adding an additional source of electron scavengers in the solution phase.

The optical absorption spectrum of solvated electrons at the plasma-liquid interface was also measured using a series of individual laser diodes.Interestingly, the interfacial spectrum is significantly blue-shifted from the well-known bulk spectrum.Furthermore, the blue tail of the bulk spectrum is not observed in the interfacial spectrum.These differences may be due to the intense electric field at the interface, which results in a Stark shift, or the relatively high concentration of Na⁺ ions, which is also known to blue-shift the spectrum.