Characterization of particle charging in low-temperature, atmospheric-pressure, flow-through plasmas

Abstract

While plasmas are now routinely employed to synthesize or remove nano- to micron-sized particles, the charge state (polarity and magnitude) of the particles remains relatively unknown. In this study, charging of nanoparticles was systematically characterized in low-temperature, atmospheric-pressure, flow-through plasmas previously applied for synthesis. Premade, charge-neutral nanoparticles of MgSO4, NaCl, and sea salt were introduced into the plasma to decouple other effects such as the reactive vapor precursor, and MgSO4 was selected as the focus because of its stability (i.e., no evaporation) in the plasma environment. The charge fraction and distribution of the particles was examined at the reactor outlet for different particle diameters (10–250 nm) as a function of plasma power and two types of power source, alternating current (AC) and radio frequency (RF). We found that the overall charge fraction increased with increasing plasma power and diameter for the RF plasma. A similar increasing trend was observed for the AC plasma with increasing particle diameter in the range of 50–250 nm, but the charge fraction increased with decreasing particle diameter in the range of 10–50 nm. The charge distribution was revealed to be bipolar, with particles supporting multiple charges for both the RF and AC plasmas, but the RF plasma produced a higher fraction of multiple charges. Differences in the characteristic timescales for particle charging in the AC and RF plasmas are a possible explanation of the trends observed in the experiments.

Enhancing charging and capture efficiency of aerosol nanoparticles using an atmospheric-pressure, flow-through RF plasma with a downstream DC bias

“…Figure 1c shows the charge distribution on MgSO 4 particles with 25 and 45 nm mean mobility diameters exiting the plasmas at two different DC voltages. At a DC voltage of 0 kV, the particles exhibit a bipolar charge distribution, which can be attributed to a two-stage charging mechanism in the plasma reactor that was previously reported (Sharma et al 2020). Briefly, particles first acquire a predominantly negative charge in the plasma volume and subsequently, in the spatial afterglow, electrons are lost faster to the walls as compared to the positive ions because of their higher mobility, resulting in a larger flux of positive ions to the particles and neutralization.…”

“…We chose the 40 nm mobility diameter particles as a representative size to estimate the ion attachment coefficient. Sharma et al (2020) reported the ion density in the same RF, flow-through, atmospheric-pressure plasma to be about 10 20 m À3 . The ion concentration in the spatial afterglow should be several orders of magnitude lower due to wall losses and electron-ion concentration recombination.…”

“…The polarity of particle charge exiting the plasma reactor was measured using a tandem differential mobility analyzer (TDMA) setup, consisting of two DMAs, a neutralizer, and a condensation particle counter (CPC). Details of the procedure have been previously reported in Sharma et al (2020). Here, we focused on the polarity of particles at two mean mobility diameters, 25 and 45 nm, as a function of the downstream DC voltage.…”

“…Ion mobility spectrometry has been applied online to flow-through plasmas at atmospheric pressure (Sankaran et al 2005) and low pressure (Chen et al 2018;Chen et al 2020) and observed the presence of bipolarly charged silicon nanoparticles after the spatial afterglow. Recently, Sharma et al (2020) systematically characterized particle charging in AC-and RF-powered flow-through, atmospheric-pressure, plasma systems. High charging efficiencies (>90%) were found for particles larger than 100 nm.…”

“…The charge distribution of particles exiting the plasma reactor was revealed to be bipolar with particles supporting multiple charges. A two-stage charging mechanism based on a characteristic time scale analysis suggested that while particles were predominantly charged negatively in the plasma volume, the relatively faster rate of loss of the electrons in the spatial afterglow (downstream of the plasma region) resulted in neutralization by positive ions, and even subsequent positive charging of some of the particles (Sharma et al 2020). The effect was found to be larger for smaller particles because of their lower initial negative charge in the plasma volume.…”