Probing the aqueous interface of plasma electrochemical reactors using in situ Raman spectroscopy*

Plasma electrochemistry (PEC) has been under investigation for potential applications such as agriculture, water treatment, decarbonized chemical processing, and nanomaterials synthesis. PEC in water leads to the formation of gas-phase species that undergo a cascade of processes in the liquid to produce aqueous species such as OH, H₂O₂, ONOOH, NO₂^-, and NO₃^- that are sought after in these applications. A major challenge facing the development of PEC-related applications is the direct and detailed experimental investigation of the plasma-liquid interfacial region. The majority of existing liquid-phase diagnostics must be performed ex situ, removed from the plasma reactor and after treatment. To extend the scope of diagnostics, we study a wide range of physical and chemical properties at the plasma-water interface using in situ spontaneous Raman microspectroscopy. For PEC in pure water, in situ Raman spectra showed that the concentrations of aqueous H₂O₂ and NO₃^- both exceed those of the bulk liquid at a depth of a few tens of microns from the plasma-liquid interface. We will study PEC in air plasma-water systems at atmospheric pressure, using both batch reactor and electrospray configurations. We will focus on the spectral profile of the –OH stretch band of water and of probe molecules such as NO₃^-. Analysis of the –OH stretch band reveals that the plasma weakens the hydrogen bonding network of water. To help pinpoint the cause, we will track the broadening of the N-O symmetric stretch mode (v₁) of NO₃^-.