Effects of nonthermal plasma jet irradiation on the selective production of H₂O₂ and NO₂ ^- in liquid water

We present the effects of the application of a nonthermal plasma jet to a liquid surface on H₂O₂ and NO₂ ^- generation in the liquid. Two distinct plasma irradiation conditions, with plasma contact and with no observable plasma contact with the liquid surface, were precisely compared. When the plasma was made to touch the liquid surface, the H₂O₂ concentration of the plasma-treated water was much higher than the NO₂ ^- concentration. In contrast, when no observable contact of the plasma with the liquid surface occurred, the ratio of the NO₂ ^- to H₂O₂ concentration became over 1 and NO₂ ^- became more dominant than H₂O₂ in the plasma-treated water. Our experiments clearly show that reactive oxygen and nitrogen species can be selectively produced in liquid using appropriate plasma-irradiation conditions of the liquid surface. The ratio of NO₂ ^- to H₂O₂ was controlled within a wide range of 0.02-1.2 simply by changing the plasma-irradiation distance from the liquid surface.