**1. Introduction**

Nanoscale particles can have structural, thermal, electromagnetic, optical and mechanical properties that are significantly different from those of larger particles [1]. Particle properties are highly size-dependent and can be exploited in a variety of applications. Therefore, the control of nanoparticle size is very important and desirable, but it also represents a challenge. Nanoparticles can be prepared using various methods, such as laser ablation [2], ultrafine bubbles and pulsed ultrasound [3], corona discharge [4] and sparking processes [5–11]. The methods and conditions used to prepare nanoparticles strongly influence their size and shape [4,12]. The sparking process is of interest because it is performed in atmospheric air, is inexpensive and does not require a vacuum system. Moreover, this method is flexible regarding the material used, can be up-scaled, and is environmentally friendly as it does not produce any waste and does not require a chemical precursor. Although there have been several publications describing the production of nanoparticles using the sparking process in atmospheric pressure, the effects of the wire electrode properties on the nanoparticle size and formation pattern of the deposited films

**Citation:** Kumpika, T.; Ruˇcman, S.; Polin, S.; Kantarak, E.; Sroila, W.; Thongsuwan, W.; Panthawan, A.; Sanmuangmoon, P.; Jhuntama, N.; Singjai, P. Studies on the Characteristics of Nanostructures Produced by Sparking Discharge Process in the Ambient Atmosphere for Air Filtration Application. *Crystals* **2021**, *11*, 140. https://doi.org/ 10.3390/cryst11020140

Academic Editor: Raghvendra Singh Yadav Received: 20 January 2021 Accepted: 26 January 2021 Published: 29 January 2021

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have not been reported previously. During the sparking process, the applied voltage induces high-temperature arcing plasma in the air gap via the field ionization process. Electrons and ions in the plasma bombarded the two tip surfaces, resulting in the vaporization and liquefaction of the metal electrodes. In our previous work [5], the formation of nanoparticles was modeled using the Young–Laplace relation by considering the relative surface energies and different pressures inside and outside the molten layers on the electrode surfaces. For the low-pressure atmosphere, Tabrizi et al. [13] produced gold nanoparticles using the spark discharge at the pressure of 1–2.5 bar. They explained that the electrode materials were evaporated, and the nanoparticles were nucleated and agglomerated from the vapor. In this report, the effects of the electrode properties on the generated nanoparticles from both vaporized and molten metals were described and applied in forms of aerosol for disposable face mask testing of filtration efficiency.
