Effects of Current on Arc Fabrication of Cu Nanoparticles

Arc-fabricated copper nanoparticles (Cu Nps) size, morphology and the crystalline structure, as well as the yields of Nps appear sensitive to the applied currents (50–160 A) in distilled water. The results indicate that the sizes of Cu Nps are directly proportional to the currents employed. At 50 A, TEM, XRD, and SEM analyses show fabrication of relatively purest, the most dispersed, facecentered cubic (fcc) brown Cu Nps with rather smallest average size of 20 nm. At the same current, the TGA-DTA analysis reveals neither weight loss nor gain, indicating thermal stability of the fabricated Cu Nps.

We have also reported syntheses of the low-cost Cu Nps and Cu 2 O Nps preparation through explosion of copper wires [26].Water introduced as the medium of choice by many reports, including our recent account of media effects on arc fabrications of nanobrass (63%Cu + 37%Zn) [27].While arc fabrication of Cu Nps in distilled water is already reported, the crucial role of current is not addressed yet.Hence, in this manuscript we adopt distilled water for probing the effects of current (50-160 A) on the arc fabrications of Cu Nps.It is found that density of current is a key factor for the morphology, controlling particle sizes, and yields of copper Nps.Increasing the current can cause to increasing the particle sizes and decreasing the yield of Nps production [28].

Experimental
Our arc method requires only a direct current (DC) power supply and commercially available copper rods.Two highpurity copper rods (95.90%) with diameters of 2.5 mm and length of 30 mm are employed as a movable anode and a static cathode in our arc discharge experiments in distilled water.The distance between the two copper electrodes is set at 1 mm with a 45 • angle between the two electrodes.Different currents (30,50,70,90,100,115,150, 160 amperes) are passed through water-submerged copper electrodes (1-10 milliseconds).The arc discharge is initiated by slowly detaching the moveable anode from the cathode.Consequently, the cathode-anode gap is controlled at approximately 1 mm to maintain a stable discharge current and average voltage of 25 V in experiments.Separating the electrodes increases the voltage, while bringing the electrodes close together decreases it.The voltages and currents employed are recorded when stable discharge conditions are attained.
The Cu electrodes are heated by the high temperature of the arc, and metal atoms are separated from the metal surface and evaporated into metal vapour.The cooled metal vapor in water lead to the formation of primary particles by nucleation mechanism turning into Cu Nps dispersed in distilled water [29].Gas bubbles are formed in the water during the arc process due to the plasma vaporization/decomposition of the anode material and water.These escaping gas bubbles act both as a condensing media and as carriers of the final products to the water surface.The growth rate of the nucleus is controlled by the concentration of the vaporized metal and the temperature of the medium [30].The Xray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are used for depiction crystalline structure, morphology, and size of Cu Nps, respectively.Thermal stability of Cu Nps is characterized by thermogravimetric and differential thermal analyses (TGA-DTA).

Results and Discussion
Our objective in this work is to find the best current for fabrication Cu Nps with the better uniformity, good particle size distribution, and the higher yield.We discussed in detail the impact of current on the metal Nps fabrication elsewhere [28].Eight different currents (30, 50, 70, 90, 100, 115, 150, and 160 A) are passed through Cu electrodes.At the employed currents ≥50 A, two distinct nanopowders (w/w 50 : 1) are obtained which become easily separated through filtration.The major nanopowder is a brown precipitate which consists of pure Cu Nps, while the minor product is a black color, water-suspended mixture of Nps, consisting of copper oxide Nps (CuO and Cu 2 O Nps).

SEM and XRD Analyses. SEM and XRD results indicate
that the less miscible, arc-fabricated, brown powders are made of pure Cu Nps, which appear as face-centered cubic (fcc) crystals (Figures 1(a)-1(g) and 2).These SEM images are in contrast to those of the starting copper electrodes, which indicate no evidence of nanostructure, prior to the arc discharge (Figure 1(h)).The visual inspection of the SEM images shows conspicuous current effects on the yield, size, and morphology of brown Cu Nps (Figure 1).Accordingly, in the distilled water, 50 A is rather the best current which produce the size-selected, single crystalline Cu Nps with the average size of 58 nm (Figure 1(a), see (S), Figure S1in Supplementary Material available online at doi: 10.1155/2010/403197).
SEM results show that increasing the current enlarges mean-sizes of the Cu Nps (Figure S1).Obviously, the higher arc currents increase the rate of anodic erosion, causing an increase in the macroparticle formations [14].At currents higher than 100 A, the yield of Nps drops while their sizes increase (Figures 1(e)-1(g), Figure S1).This is due to the higher rate of vaporization of copper atoms at higher currents, making the growth rate of particles higher leading to larger particle size [31].As a result, the sizes of Cu Nps are directly proportional to the currents employed (Figure 3).While changing current has significant effects on the particle size, it does not show any noticeable impact on the Cu Nps compositions.Hence, brown Cu Nps, formed in distilled water, at different currents, show XRD lines (111), (200), and (220) at 2θ = 43.29 • , 50.43 • , 74.10 • , respectively (Figure 2).
In other words, at all currents positions and intensities of XRD peaks are similar, suggesting arc fabrications of pure Cu Nps.At all the employed currents, a watery black nanopowder is produced as a byproduct (w/w 1 : 50), in addition to the arc fabricated brown Cu Nps (Figure S2).The XRD patterns of the brown Cu Nps, fabricated at different currents, do not change after one-month storages in the open air, or distilled water.In contrast, one month storages of the "black nanopowders", in distilled water, induce changes in their XRD patterns, reflecting further oxidation of Nps (Figure S3b).We obtained merely the brown nanopowder in the arc fabrication experiment using a PVP aqueous medium (w/v 1 : 1), at 50 A [13,32].Due to coordination between Cu and PVP, the oxidation of Cu Nps is avoided.Its XRD and SEM results show pure Cu Nps with average grain size of 50 nm, respectively (Figure S4).

TEM Analysis.
Using the current of 50 A (current of choice), TEM images of Cu Nps illustrate their spherical morphologies, confirming the SEM images (Figures 4 and  1(a)).To the best of our knowledge, this is the first report on the arc synthesis of metallic Cu Nps with such a fine particle size.Assuming that an Np is spherical, the average diameter of Nps is estimated to be about 20 nm by averaging the diameters of the 50 particles measured in several directions in the TEM image, which is remarkably different with those primarily estimated through SEM (58 nm) analysis, indicating high precision of TEM apparatus.However, small particles aggregate into second particles because of their extremely small dimensions and high surface energy (Figure 4(a)).The selected-area electron diffraction (SAED) pattern displays that the Nps are multiple-surface orientated consisting mainly of Cu atoms with a face-center cubic (fcc) structure (Figure 4(b)).The three characteristic diffraction rings are made of diffraction spots corresponding to (111), (200), and (220) faces of the fcc Cu crystal from the inside to the outside diffraction ring, respectively.This result confirms XRD findings (Figure 2).

TGA-DTA Analysis.
Decomposition kinetics and thermal stabilities are deduced through TGA-DTA, on 5 mg samples of the preheated Cu Nps (arc fabricated at 50 A, as the best current) (Figure 5).Rather rapid weight decrease is observed at 25-100 • C, due to vaporization of the residual

Conclusion
Cu Nps are prepared in a large scale arc discharging with homemade apparatus at different currents (50-160 A).Density of current is found as a key factor for the morphology, controlling particle sizes, and yields of Cu Nps.It is found that decreasing the current results in a substantial decrease in the particle size.According to SEM results, the trend of Cu Nps size is proportional to working current showing 50 A (58 ± 9 nm) > 70 A (74 ± 12 nm) > 90 A (97 ± 17 nm) > 100 A (131 ± 20 nm).50 A appear the best current for fabrication of pure, small, and high yield of Cu Nps.The TEM images show that the Cu Nps are spherical with a narrow particle size distribution and an average particle size of 20 nm (58 nm indicated by SEM).Its XRD and SEAD results indicate the fcc structure of synthesized Cu Nps.Changing current has significant effects on the particle size, while it does not show impact on the Cu Nps compositions.

Figure 1 :
Figure 1: SEM images showing the effects of seven different currents (50-160 A) on the arc fabricated brown copper nanoparticles, Cu Nps (a)-(g), along with the starting copper rod image, prior to the arc discharge (h).

Figure 3 :
Figure 3: Variation of as-produced Cu Nps sizes as a function of employed currents (50-160 A).

Figure 4 :
Figure 4: TEM image (a) and the selected-area electron diffraction pattern (b), of Cu Nps synthesized by arc discharge in distilled water, at 50 A.

Figure 5 :
Figure 5: TGA-DTA plots of Cu Nps arc fabrication of 50 A, in distilled water, at a 20-800 • C range with a rate of 10 • C /min, using air atmosphere.