Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface

Abstract

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 µs), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons' absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO2(aq)), nitrate (NO3(aq)), hydrogen peroxide (H2O2(aq)), and dissolved carbon dioxide (CO2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.

Effect of Competing Oxidizing Reactions and Transport Limitation on the Faradaic Efficiency in Plasma Electrolysis

“…6 More recently, it was argued that in the plasma cathode configuration, electrons from the plasma must be injected into the liquid in order for current to be maintained, 7 and this was ultimately confirmed by optically detecting plasma-injected solvated electrons via absorption spectroscopy. 8,9 Notably, it was found that plasma-injected solvated electrons have essentially the same properties as those produced by radiolysis, such as the rate constants for various reactions reported in the radiation chemistry literature. 10,11 Solvated electrons (e − aq ), commonly called hydrated electrons when in aqueous solutions, are one of the most powerful reductants with a reduction potential of −2.88 V vs. NHE, 12 and can be used to reduce a wide variety of species.…”

An investigation of evanescent wave-induced fluorescence spectroscopy for exploring high refractive index media

“…In a conventional evanescent wave experiment, the first medium (TIR substrate) is generally taken as a glass (e.g. silica, SiO 2 ), although such measurements can also be performed at the interface between two liquids [15], a plasma and a liquid [16] or a liquid and a gas [17]. Evanescent wave excitation of the object is possible while the refractive index, n, of the sample does not exceed that of the glass (n = 1.47 at 400 nm for silica [18]).…”

Experimental confirmation of solvated electron concentration and penetration scaling at a plasma–liquid interface

“…Solvated electrons in plasma electrochemical systems were first directly detected using a reflection-based absorption measurement now termed total internal reflection absorption spectroscopy (TIRAS) [3,13], and a basic reaction model was used to estimate their penetration depth to be l = 12.3 ± 4.9 nm [3]. (Note that is larger than the value of l = 2.5 ± 1.0 nm in the original publication of [3], but was corrected in a corrigendum due to an error in the analysis.)…”

“…In this letter, we extend the scaling theory in equation (3) to develop a predicted scaling for the TIRAS signal with current density and experimentally confirm this predicted scaling, from which we can draw additional insight into the reaction and penetration of e -(aq) . TIRAS, illustrated in figure 1, uses differential absorption spectroscopy to detect solvated electrons at the plasma-liquid interface [13]. A negative polarity, DC glow discharge is generated between a hollow, sharp-tipped steel electrode (cathode) and the surface of a liquid electrolyte in a glass experimental cell flushed with an argon (Ar) atmosphere.…”

“…Using equation (7), the ionic strength was converted to current density in order to verify the scaling in equation ( 6), assuming that the plasma radius at j high is the same as a DC plasma at the same current. At each concentration, multiple TIRAS measurements (between 5-10) were taken following our standard procedure [13]. While our prior work used a diode laser [13], here we used a tunable Ti:Sapphire laser (Matisse 2, Spectraphysics) that produces an overall lower level of noise and more consistent signal-to-noise ratios of ∼100/1.…”