Water activation using Ar-H$_2$ atmospheric pressure plasma jets

TL;DR

The study compares two atmospheric pressure plasma jets fed with Ar and Ar-H$_2$ mixtures to understand how excitation waveform and gas composition influence discharge properties, plasma chemistry, and water activation. Optical emission spectroscopy, alongside electrical and thermal diagnostics, shows that electron density can reach $n_e$ on the order of $10^{21}$–$10^{22}$ m$^{-3}$ as H$_2$ is added, while gas and rotational temperatures depend strongly on the power source. Water treatment reveals that Ar plasmas favor NO$_2^-$, NO$_3^-$, and H$_2$O$_2$ production, whereas Ar-H$_2$ plasmas enable NH$_3$ and N$_2$O formation in the water, suggesting agricultural applications with NH$_3$ and a caution for medical use due to ammonia toxicity. The results demonstrate that plasma source frequency and power shape RONS output, enabling targeted tailoring of plasma-activated water for different applications and highlighting the need for parameter optimization to balance beneficial versus harmful species.

Abstract

Whether for materials processing or medical applications, the use of atmospheric pressure plasma jets (APPJs) has emerged as a relevant alternative to conventional methods. Within the APPJs research field, the search for innovation aims not only to solve existing problems but also to explore novel options for generating plasma jets and find new possible applications. In this work, the properties of Ar-H$_2$ APPJs generated using two plasma sources that differ in the frequency, amplitude, and waveform of the generated voltage signal were studied through electrical, thermal, and optical characterization. The discharge parameters were analyzed as a function of the H$_2$ content in the gas mixture, with this parameter varying from 0\% to 3.5\%. Optical emission spectroscopy revealed that the same reactive species were produced for both plasma sources, except nitric oxide (NO), which was observed only for the source operated at a higher frequency (PS #1). Applications for water activation were performed without H$_2$ and with 3.5% H$_2$ in the gas mixture. The results of water treatment revealed that ammonia is also produced when H$_2$ is added to the working gas. This finding suggests that the water treated by a Ar-H$_2$ plasma jet can be an attractive option for use in agriculture.

Figures (11)

• Figure 1: Experimental arrangement. (a) Overview and electrical measurement scheme. (b) Reactor configuration for plasma sources #1 and #2,eft and right, respectively. (c) Setup for gas temperature measurements. (d) OES measurement scheme. The elements in the figures are out of scale. The value of $d$ was 10.0 mm in all experiments carried out in this work.
• Figure 2: Voltage ($V$) and current ($i$) waveforms measured using $\rm{Ar}$ and $\rm{Ar + H_2}$ for (a) PS #1 and (b) for PS #2, respectively.
• Figure 3: Overview of the optical emission spectra in the wavelength range from 200 nm to 758 nm measured for plasma sources (a) #1 and (b) #2. Detailed spectra from 200 nm to 410 nm were obtained for plasma sources #1 and #2, (c) and (d), respectively. Curves for three different $\rm{H_2}$ percentages are shown in each figure. The spectra in (a) and (b) were measured with the Avantes spectrometer, while the spectra in (c) and (d) were measured using the Horiba spectrometer. Each spectrum shown in (c) and (d) is composed of two sets of measurements made between 200-300 nm and 300-410 nm. The asterisks in (c) and (d) denote $\rm{Ar~I}$ine emissions. The $\dagger$ in (d) indicates an overlap of multiple emitting species.
• Figure 4: Radiation intensity of selected species as a function of the hydrogen content in the gas mixture for (a) PS #1 and (b) PS #2. The ratio between the intensity of $\rm{H_{\alpha}}$ and $\rm{Ar~I}$ (727 nm)ine emissions is also plotted in theowest frame of (a) and (b). OES measurements were performed with the Horiba spectrometer.
• Figure 5: Overview of the UV-Vis absorption spectra of PTW samples using $\rm{Ar}$ and $\rm{Ar + H_2}$ with PS #1 for 5 and 10 min plasma exposure, and with PS #2 for 10 min. The plasma-treated water volume was 30 ml in all cases.