Preparation of silver nanoparticles in a high voltage AC arc in water

The article presents for the first time the synthesis of silver nanoparticles in an electric arc of high-voltage alternating current with a frequency of 50 Hz. In particular, the method and apparatus necessary for the preparation of nanoparticles in water solution is discussed. Current–voltage characteristics depending on the mutual distance between the electrodes are presented which show a very high stability of the generated discharge phenomena. The obtained nanoparticles were examined using various analytical techniques such as UV–Vis spectroscopy, dynamic light scattering (DLS), zeta potential, energy dispersive X-Ray analysis (EDS), X-ray diffraction (XRD), and X-ray fluorescence (EDXRF). The morphology, surface and size of the obtained nanoparticles was carried out using transmission electron microscopy (TEM) and scanning TEM (STEM) equipped with the annual dark-field imaging scanning atomic-scale chemical mapping (STEM). The designed simple power supply unit consisting of an autotransformer and a microwave oven transformer (MOT) makes the preparation of silver nanoparticles both simple and economical.


Introduction
Due to their biocidal, antibacterial and antiviral properties, silver nanoparticles are widely used in medicine and health care [1][2][3][4], although the applications in other fields of science and technology, such as in chemical analysis (SERS) [5][6][7] and electronics [8] are also very significant. Their specific uses strictly depend on the method of synthesis which include: inert gas condensation [9][10][11][12][13][14], radiolysis [15][16][17], sol-gel method [18][19][20] ion implantation [21], chemical vapour deposition (CVD) [22], polymerization [23,24] or synthesis by chemical reduction from silver salts and organometallic precursors [25]. These commonly known methods belong to bottom-up approach of nanoparticles synthesis. The submerged arc discharge synthesis method described here, together with e.g. laser ablation [26,27], belongs in turn to the top-down methodology in nanoscience and is considered both the simplest and the cheapest. It consists in creating an arc discharge between the silver electrodes immersed in a liquid medium. The generated high temperature associated with the shortcircuit current flow leads to ablation of the bulk material of the electrodes with a formation of small metal objects [28][29][30][31][32]. At present, most solutions for generating an arc between electrodes are based on the use of direct current [33]. Unfortunately, such a solution is associated with a quite expensive electric current rectification system to maintain the stability of the power supply. In this communication, instead of a low-voltage rectifier supply, we propose a solution based on the generation of a high-voltage arc in which the synthesis of silver nanoparticles also takes place. We observed that this solution accelerates the process of producing nanoparticles due to the stabilization the electric arc between the electrodes. To the best of our knowledge the presented solution based on MOT transformer seems to be the simplest method of making high voltage AC arc thanks to appropriate power systems, creates a stable arc between the electrodes and it is powerful, and cost effective.

Materials
Silver rods with a purity of ≥ 99.99% and a diameter of 2 mm were purchased from Mint-Metals Poland and were used as electrodes. The submerged arc process was carried out in a distilled water with a resistivity of 18.2 Mohm/cm.

Analysis methods
The voltage parameters were measured using a DP-100 high-voltage differential probe with the following parameters: accuracy ± 2%, the bandwidth of the DC 3 dB is 100 MHz, differential input impedance 54 Mohm with a 3.5 ns rise time. The current measurement was made using a PA-677 probe with a DC-1 MHz range up to 70 A. The signals were monitored with a Rigol MSO1074Z oscilloscope.
The absorbance spectrum of the colloidal sample was measured in the range of 320-900 nm, using a UV-Vis spectrometer Shimadzu-UV 1240 with distilled water as a reference. The size distribution and stability of AgNPs were performed by measuring DLS and Zeta potential measurements using Malvern Zetasizer Nano ZS90 instrument. X-ray diffraction (XRD) analysis was conducted on a DRON diffractometer (Russia) using monochromatic Cu Kα radiation (λ = 1.5406 Å) operated at 50 kV and 40 mA at the accusation scan θ-2θ. The scanning was done in the 2θ range of 5°-70°. The morphology, shape and size of the silver nanoparticles were examined by TEM technique with a Talos F200X. Atomic-scale chemical maps and energydispersive X-ray spectra (EDS) were obtained using scanning transmission electron microscopy STEM-EDS/JEOL JEM 2800. Elemental analysis chemical was performed using the EDXRF technique on an Epsilon 1 portable XRF Analyzer from Malvern Pananalytical.

Experimental details
The process of producing silver nanoparticles takes place in the current arc between two silver electrodes placed opposite each other at an angle of 60°. The electrodes were 50 mm long and 2 mm in diameter and were immersed in water to a depth of 2 cm. Figure 1a shows a scheme of glassy reactor for the synthesis of nanoparticles a glassy vessel is filled with distilled water and equipped with a magnetic stirrer for uniform temperature distribution of the water medium and homogeneous distribution of the produced nanoparticles. The high voltage alternating current arc was generated using a power supply (Fig. 1b). It consists of an autotransformer with an adjustable output voltage in the range of 0-230 V, which supplies the highvoltage transformer typically used in microwave ovens, so-called MOT, and it allows the generation of an output voltage between the electrodes in the range from 0 to 2 kV. The stepper motor precisely controls the position of the electrodes relative to each other. The measured distance between the electrode tips optimal for generating the electric arc of the discharge was 1.2 mm.
In the experiments, the output voltage of the autotransformer was set at 100 V, which generated a voltage of 1100 V on the secondary winding of the high voltage transformer (MOT). The process of synthesis of silver particles was carried out in the electric arc in two time regimes-for 5 min (sample A) and 20 min (sample B). The temperature of the 250 mL water medium was not stabilized during the arc. Thereby it rose in the reaction vessel from 19 °C before the process to 32.5 and 63 °C during the 5 and 20 min arc, respectively. The loss of weight of the electrodes is not uniform and was 0.0013 mg for the first one and 0.0011 mg for the second in a 5 min process. For a 20 min arc, the weight loss of the electrodes was 0.0044 g and 0.0033 g, respectively. Thus the overall weight loss of the silver electrodes was 0.0024 g and 0.0077 g for the arc durations used. From the first mass of silver, 5.6 10 14 spherical particles with a diameter of 92 nm can be obtained and from the second one 1.80 10 15 NPs. Figure 2 shows the current-voltage waveforms during NPs synthesis in an AC arc. The open circuit voltage between the silver electrodes before starting the process was 1.1 kV AC. During the process, the maximum amplitude of the voltage between the electrodes decreases to 850 V (1700 V peak-to-peak voltage), and maximum current was set to 0.8 A (1.6 A peak-to-peak current).

UV-Vis spectral analysis
The Ag nanoparticles concentration was controlled with the arc discharge time, which was 5 and 20 min. The formation of the Ag NPs during the HV AC process was followed by change in the colour of the reaction solution from colourless by yellow to dark orange, as illustrate photographs in Fig. 3. The UV-Vis absorption spectra of the Ag NPs with different concentrations are shown in Fig. 4. The broad band with a maximum at 404 nm (sample A) derives from the phenomenon of surface plasmon resonance characteristic of silver nanoparticles [33]. The profile of the band with long absorption tail suggest the presence of not only spherical nanoparticles in the water but objects of various shapes. For sample B, the spectrum position slightly shifts to 406 nm and widens at ~ 500 nm, indicating a minor change in the size distribution of the NPs.

Dynamic light scattering (DLS) and zeta potential results
The size distribution profile of the synthesized silver nanoparticles measured by the DLS method is shown in Fig. 5. The scattered signal confirms the broad particle size distribution which agrees well with the unsymmetrical extinction peak in the long wavelength absorption region observed in the UV-Vis spectra, especially for sample B. Size distribution profiles reveal two populations of NPs for each sample with the average size around of 18 and 90 nm. The shorter synthesized colloid A shows a narrower size distribution. Also the zeta  (Fig. 6). The calculated average particle size is 48.9 nm and 46.8 for a 5 min and 20 min sample, respectively. In turn, the zeta potential of the synthesized Ag NPs is a sharp peak at − 20.4 for sample A, and − 22.31 mV for sample B (Fig. 6), but for the last one it is wider. These values suggests that the surface of the silver nanoparticles dispersed in water medium is positively charged.
High negative values are responsible for the repulsion between the particles and make the colloid very stable for at least months.

Transmission electron microscope (TEM)
TEM images of the colloidal silver nanoparticles obtained in the HV AC arc discharge confirm their heterogeneity. Interestingly, they also show the lack of aggregates or large particles observed in syntheses by other methods in the current arc [34]. Although nanoparticles are mostly oval in shape (Fig. 7a), triangular or trapezoidal nanoparticles are also clearly visible (Fig. 7b). The measured particles size varies from 20 to 100 nm, but their shape is not related to a specific size. Interestingly, TEM photographs taken at higher magnifications reveal a 4 nm thick shell around the obtained nanostructures.

Chemical maps of Ag NPs on the atomic-scale by high-angle annular dark-field imaging (HAADF) using a scanning transmission electron microscope (STEM) technique
The atomic composition and morphology of the synthesized NPs as well as the thin coating around NPs visible at higher magnifications (Fig. 7) was examined by the STEM method. Figure   During the discharge in HV alternating current arc, dissociation of water molecules with the formation of reactive H • and OH • radicals and dissociation of oxygen molecules dissolved in the aqueous medium with the formation of singlet oxygen may occur. Such species, due to their high reactivity, can be trapped on the Ag surface and finally stabilize the colloid electrostatically. Energy dispersive spectrum of the synthesized Ag nanoparticles also shows the presence of silver (strong signal peak at 3 keV) as the basic component and cooper at 8 keV. The remaining elements such as O and Si, although were registered qualitatively are at trace level. The carbon atoms are the result of a thin amorphous layer covering the copper mesh.

X-Ray fluorescence (XRF) measurements
The XRF technique has been used complementary to EDS/ STEM to determine elemental composition of the silver colloid. Again, the presence of dominant Ag element is confirmed on XRF spectrum (Fig. 9). The spectrum contains peaks of silver: K α line at 22.16 keV and K β line at 24.9 keV. Also characteristic silver L-line was observed with the L-absorption edge at 3.82 keV. In addition to the silver element, Si was also detected in trace amounts.

X-Ray diffraction (XRD) Studies
X-ray diffraction analysis is very important for determination the structure of the nanocrystals and their morphology, as the nanoparticles have been produced in non-equilibrium conditions of very high temperature and ionizing voltage. Figure 10 shows the XRD pattern of the obtained nanoparticles. The diffraction clearly shows the characteristics reflection 2θ peaks at 38. 2, 44.4, and 64.6° corresponding to the (111), (200), (220) planes of fcc lattice of metallic silver, respectively [35], [36]. The average crystalline size of the silver nanoparticles was estimated using the Debye-Scherrer's equation (Eq. 1) on the basis of determining the width of the peak (111) in Bragg reflection.
where D is the mean size of the crystallites, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) in radians, 0.9 is the value of the shape factor and θ is the Bragg angle. The estimated average size (1) D = 0.9 cos Fig. 6 The Zeta potential of AgNPs colloidal solutions of sample A and sample B of the silver crystalline domains is 9 nm and is smaller than the size of nanoparticles observed from TEM images. This means that the nanoparticles growing in the HV discharge arc are made of smaller crystallites.

Summary
The presented solution consists in omitting the electric current rectification system in the process of electric arc generation for the synthesis of metal particles. Instead, the use of a high-voltage alternating current arc with a frequency of 50 Hz can be successfully used to produce silver nanoparticles. During the current flow, the potential amplitude between the silver electrodes was set to 850 V. The colloid is stable for at least several months under ambient conditions. TEM images show silver nanostructures of various shapes and sizes including spheres, triangles and polygons of the sizes between 10 and 120 nm. The presence of Si and O atoms in the solution and as the thin coating on Ag NP surface is intriguing. Accelerated electrons and ions at such a high voltage may have enough energy to etch silica walls of the reactor vessel. This is only a hypothesis that should be verified. In this regard, more study is needed to elucidate the role of voltage and frequency parameters during the whole process of syntheses on the shape and size distribution. Further research should lead to a detailed explanation of the nanoparticle stabilization effect including both electrostatic as well as steric factors by introducing to the submerged medium various stabilizing agents. The new method of particle synthesis may also apply to other metals and semiconductors from which the electrodes will be made.

Conflict of interest
The authors declares that they have no conflicts of interests.
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