**2. Experimental**

To study the size of nanoparticles, various metal wires were used as sparking tips. The metals used were zinc (d = 0.38 mm), aluminum, silver, gold, nickel, cobalt, titanium, vanadium and molybdenum (purity ≥ 99.5%, d = 0.25 mm, Advent Research Materials Ltd., Eynsham, Oxford, England). The sparking tips were placed horizontally, 5 mm above the center of the glass substrate, with a gap of 2 mm between them. Sparks were produced by the discharge of a 25 nF capacitor at 10 kV. Before the nanoparticles were formed on a glass slide substrate (10 <sup>×</sup> <sup>10</sup> <sup>×</sup> 1 mm<sup>3</sup> ), the substrate was sonically cleaned in acetone, ethanol and distilled water and dried under nitrogen gas. To eliminate the effect of initial conditions, the tips were sparked 100 times before collected the nanoparticles. The spark was created once in an ambient air at atmospheric pressure to study the deposited particle sizes.

Scheme of sparking discharge apparatus is represented in Figure 1. The Supplementary Information depicts all of the sparking apparatus used in our research; from the first prototype that was used for the fabrication of nanoparticles for carbon nanotube growing, to this study. The principle of the work was described previously [14], in brief, our research technique consisted of a power supply that was connected to a 220 V alternative current source. The power supply unit consisted of an AC/DC converter and a controlled source of voltage and current, which was connected to a 7 KV DC boost step-up power module high-voltage generator, which was in turn connected to a capacitor that directs electrical power to a circuit breaker with changeable heads that hold metal wires. These wires were placed in a sealed container, through which gasses flowed, and in which a substrate for nanoparticle collection was placed. *Crystals* **2021**, *11*, x FOR PEER REVIEW 3 of 14 sparking a very low surface coverage of nanoparticles on a substrate for AFM. After approximately one second of sparking onto the substrate, we noticed films with primary particles as an isolated island, consisting of primary particles that were deposited very far

> Nanoparticles produced using various sparking metals were imaged using AFM, as shown in Figure 2a. Because the lateral (horizontal) sizes of the nanoparticles estimated by AFM show a broadening effect due to a tip shape convolution [15], the nanoparticle sizes were determined by measuring their vertical heights. Due to the random nature of the nanoparticle agglomeration, some particles attached to others and created a secondary particle deposition on the substrate. To determine the primary size of the particles, only primary particles that appeared in the images were used. It can be clearly seen, that a low energy level is needed to melt the metals (Zn, Ag, Au), resulting in a higher density of the particles in the area under observation. The reason for this is that these metals require a lower energy to melt the electrode, and in turn, to melt a nano-droplet. The energy applied to the tips (*Eapp*) is separated to the energy lost to the environment (*Eloss*) and the energy

> > ( −

) + ] (1)

, whereas the high heat

. Moreover, aluminum

away from the other particles.

**3. Results and Discussions** 

**Figure 1.** Conceptual representation of sparking machine apparatus. For pictures of the machine please see the Supplementary information. **Figure 1.** Conceptual representation of sparking machine apparatus. For pictures of the machine please see the Supplementary Information.

used to melt the metal tips, as described by Equation (1)

noscope III (Veeco, New York, NY, USA) software.

aluminum having a remarkably high heat fusion of 0.90 Jg−1K−<sup>1</sup>

fusions of zinc, silver and gold are in the range of 0.13–0.39 Jg−1K−<sup>1</sup>

= + [

where *c<sup>p</sup>* is the specific heat capacity, m is the effective mass of the metal tip, *T<sup>m</sup>* is the melting point, *T<sup>r</sup>* is the room temperature and *L<sup>f</sup>* is the heat of fusion. As shown in Table 1, the melting point of aluminum (933 K) is lower than that of silver (1235 K) and gold (1337 K), however, there a reduced amount of sparked aluminum observed. This is due to

is always covered with a microscopic thin layer of Al2O<sup>3</sup> that is extremely hard and inert. Therefore, this oxide also reduces the creation of nanoparticles during the discharge. Figure 2b shows the height measurement of zinc nanoparticle using section analysis of Na-

To study the particle sputtering patterns resulting from the sparking process (and therefore to study the directions in which the particles were expelled from the electrode wire's melting surface), gold and cobalt wires were used as the sparking electrodes, since they produce sparked nanoparticles of easily observable color. A voltage of 10 kV was applied across a 1 mm gap, causing the spark discharge. White paper substrate was placed 5 mm below the sparking gap and examined after sparking 1000 times.

The sparked electrodes and the deposited nanoparticles were characterized using scanning electron microscopy (SEM, JEOL JSM 6335F, Akishima, Japan). The Raman spectra were obtained with a 514.5 nm argon ion laser at a room temperature (Jobin Yvon Horiba T64000, Chiyoda-ku, Japan) to determine crystallinity of the films.

Surface characterization was done by using an atomic force microscope (AFM) in the tapping mode (Digital Instruments, Inc., Santa Barbara, CA) equipped with a standard Si tip and operated at a scan size of 1 x 1 µm<sup>2</sup> in air at room temperature. Section analysis was carried out to estimate sizes of the NDs (nanodots) using the Nanoscope IIIa 5.12 r3 (Veeco, New York, NY, USA) software. The primary particles size can be measured by sparking a very low surface coverage of nanoparticles on a substrate for AFM. After approximately one second of sparking onto the substrate, we noticed films with primary particles as an isolated island, consisting of primary particles that were deposited very far away from the other particles.
