Copper and Nickel Nanoparticles: Synthesis by Electrochemical

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Abstract— The unique optical, electronic, chemical and biological properties of metal nanoparticles ... voltage of an electrochemical cell is gradually raised. Beyond a ... The duration of each experiment is fixed to two hours. TABLE I. SET OF ...
Copper and Nickel Nanoparticles: Synthesis by Electrochemical Discharges Anis Allagui, Rolf Wüthrich

Elena A. Baranova

Department of Mechanical and Industrial Engineering Concordia University Montreal, QC, Canada [email protected]

Department of Chemical and Biological Engineering University of Ottawa Ottawa, ON, Canada [email protected]

Abstract— The unique optical, electronic, chemical and biological properties of metal nanoparticles have stimulated a large research activity into manufacturing methods of these nanostructures. An example of copper and nickel nanoparticle synthesis using electrochemical discharges in aqueous solutions is described. This low-cost template technique is a facile procedure to fabricate large quantities of metallic nano-sized particles. The resulting particles are characterized using an electrochemical method (cyclic voltammetry), Energy Dispersive X-ray Spectroscopy (EDS) analysis and Transmission Electron Microscopy (TEM). Nanoparticles, Electrochemical discharge, Cyclic voltammetry

I.

INTRODUCTION

There are a number of techniques involving different fields of science for nanoparticle synthesis. Among them is the gas phase synthesis of nanoparticles, developed in 1989 by Gleiter [1], which is based on nucleation and growth in a subatmospheric inert gas environment. Another technique is the sol-gel processing which is a wet chemical approach that uses the principle of gelation-precipitation followed by hydrothermal treatment [2]. Nanoparticles can also be synthesized by sonochemical processing where acoustic cavitations produces a transient localized hot zone with very high temperature and pressure gradients as shown by Sunstrom, Moser and Marshik-Guerts [3]. While controlling the very low interfacial tension through the addition of a cosurfactant, Kishida [4], in 1995, used the microemulsions principle for the synthesis of metallic nanoparticles. Recently the use of electrochemical discharges for the synthesis of nano-sized metallic particles was introduced by Lal, Bleuler and Wüthrich [5]. The authors demonstrated a way to produce platinum and gold nanoparticles by applying a fixed cathodic potential to the working electrode (WE), which is dipped into an aqueous electrolyte containing metal ions. Salts of platinum and gold, which are much less concentrated than the supporting electrolyte, were used as the electrochemical solution. The counter electrode (CE) was of larger surface than the working one. The mechanism governing the nanoparticle formation by this method is still under investigation. The first observation of light emission in an electrolyte is associated with the two French physicists Foucault and Fizeau in 1844 [6]. While they were experimenting with water electrolysis, using two electrodes of platinum under a constant

potential difference, they discovered the electrochemical discharge phenomenon. The light source and heat production properties of this phenomenon inspired a large amount of research into the area by physicists. In the middle of the 20th century the same observations were reported by Hickling and Ingram [7] during their study of glow discharge electrolysis. The phenomenon observed happens when the terminal voltage of an electrochemical cell is gradually raised. Beyond a critical voltage, electrochemical discharges are generated between the smaller electrode and the electrolyte through a gas film surrounding the electrode [8]. In recent times, engineers have applied this phenomenon, under the name of Spark Assisted Chemical Engraving (SACE) [9], to micro-scale precision machining of biocompatible materials such as glass, composites and some ceramics. In this contribution, nano-sized metal particles of copper and nickel are synthesized by electrochemical discharges and characterized. II.

EXPERIMENTAL

A two electrode electrochemical cell is used (Fig. 1).

Figure 1. Experimental setup

The eight conducted experiments are summarized in Table 1. In all experiments the working electrode (cathode) is a 0.4 mm diameter cylinder and the counter electrode (anode) is a 5x5 cm2 metal sheet, both purchased from Goodfellow. The distance separating both electrodes is fixed to 2 cm. The sulfuric acid is at 99.999% purity based on trace metals and, both nickel sulfate anhydrous and copper sulfate pentahydrate are of 99.99% purity. All chemicals used in the set of

experiments are purchased from Sigma Aldrich Canada. The applied potential difference between the WE and the CE is 32 VDC. The duration of each experiment is fixed to two hours. TABLE I.

SET OF EXPERIMENTS FOR NANO-PARTICLES PRODUCTION USING ELECTROCHEMICAL DISCHARGE

# 1 2 3 4 5 6 7 8

Electrolyte solution 1M H2SO4 1M H2SO4 1mM CuSO4.5H2O + 1M H2SO4 1mM Ni2SO4 + 1M H2SO4 1mM CuSO4.5H2O + 1mM Ni2SO4 + 1M H2SO4 1mM CuSO4.5H2O + 1mM Ni2SO4 + 1M H2SO4 1mM CuSO4.5H2O + 1mM Ni2SO4 + 1M H2SO4 1mM CuSO4.5H2O + 1mM Ni2SO4 + 1M H2SO4

WE

CE

Cu Ni Cu Ni Cu

Cu Ni Cu Ni Cu

Resulting particle Cu Ni Cu Ni Cu-Ni

Ni

Ni

Cu-Ni

Ni

Cu

Cu-Ni

Cu

Ni

Cu-Ni

Electrochemical characterizations were carried out in a three-electrode configuration cell using NaCl as a supporting electrolyte at unit molarity. Ag/AgCl electrode and a large surface area platinum foil (99.99% purity) served as both reference and counter electrode, respectively. All potentials are referenced vs. Ag/AgCl electrode. A graphite electrode was used as a working electrode. Prior to each experiment, the graphite electrode was immersed 10 times into the appropriate solution containing nanoparticles for 10 seconds and then dried with a dryer for 2 minutes. The current response of the resulting electrodes was recorded before each dipping-drying procedure in order to evaluate the amounts of the deposited nanoparticles. Cyclic voltammograms were recorded at the scan rate of 100 mV/s.

III.

RESULT AND DISCUSSION

A. TEM characterization Fig. 2 shows micrographs of the synthesized nanoparticles using a Joel JEM-2100F transmission electron microscope (TEM) - 200kV class at room temperature. Samples from experiments numbered 3 to 8 in Table 1, which correspond to micrographs "a" to "f" respectively in Fig. 2, were examined. The identification of the particles’ composition was determined systematically by energy dispersive X-ray spectroscopy (EDS). It can be seen that the copper nanoparticles in Fig. 2-a are in the form of round dots stuck together, but not well defined. The particle size ranges from 10 to less than 50 nm in diameter and some of them have coalesced with each other. Factors which determine the size distribution of nano-sized particles synthesized by electrochemical discharge include, but are not limited to, the potential difference applied between the WE and the CE, the rotation speed of the WE [5], the chemical and geometrical parameters of the electrochemical cell and the duration of the experiment. Fig. 2-b shows that the shape of the nickel particles is predominantly spherical. The diameter of the particles varies from 2 to 20 nm. EDS analysis of the 5th sample illustrates two major X-ray energy peaks at low and high energies corresponding to the copper element. Some traces of nickel are also observed within the EDS spectrum. Particle size ranges from few to dozens of nanometers. The same remark holds true for the 6th sample (Fig. 2-d); however, the two peaks correspond to the nickel element, while some traces of copper are detected. In the 7th sample, where the CE is made of copper, both populations of nanoparticles were identified with a large response of the copper. The same conclusion applies to the last sample where the presence of nickel and some additional traces of copper were identified by mean of EDS.

50 nm

a

5 nm

b

20 nm

c

50 nm

d

50 nm

e

20 nm

f

Figure 2. TEM micrographs of syntesized nanoparticles, (a) to (f) respectively correspond to experiments 3 to 8 from table 1.

As an illustration, Fig. 3 provides an EDS analysis of a sample from the first experiment in Table 1. It shows the presence of copper, as well as elements which compose the supporting electrolyte (Sulfur and oxygen). The low energy peak, which corresponds to carbon, is likely due to the surrounding atmosphere. The scope of this contribution does not include the extraction of nanoparticles from the supporting electrolyte. This will be investigated in future experiments.

appearance of a double anodic peak; while a cathodic peak is defined upon the reverse potential scan. Comparing this sample with that of experiment 3 (Table 1), which contains additional copper sulfate pentahydrate at 1 mM, one can notice a coupled distortion-amplification of the two peaks during the oxidation phase, and of the peak during the reduction process.

Figure 3. EDX spectrum of sample from experiment 1 (Table 1)

B. Cyclic voltammetry Cyclic voltammetry can provide an important insight into the chemical composition of the studied surfaces. The electrochemical characterization has been carried out on the synthesized monometallic particles of copper and nickel, as well as on the bimetallic Cu-Ni nanoparticles prepared in the present work (Table 1). The mentioned nanoparticles have been deposited on graphite electrodes prior to the electro-analytical characterization. The cyclic voltammograms (CVs) for Cu and Ni nanoparticles are presented and compared to the CVs of the corresponding bulk materials. Fig. 4 shows the CV in 1 M of sodium chloride at 100 mV/s scan rate of the graphite electrode only. It does not have any particular oxidation or reduction processes in the potential window of interest.

Figure 5. Cyclic voltammograms of (a) pure copper, (b) and (c) graphite supported nanoparticles prepared by electrochemical discharge (experiments 1 and 3 in Table 1, respectively) in 1 M NaCl at 100 mV/s scan rate

Figure 4. Cyclic voltammogram of graphite electrode in 1 M NaCl at 100 mV/s scan rate

The cyclic voltammogram of pure copper shows two oxidation peaks and one reduction peak (Fig 5 (a)). The first oxidation peak at -0.3 V in anodic scan corresponds to the oxidation of Cu0 to Cu+ and the formation of low solubility copper chloride (CuCl) compound. The second anodic peak, at -0.06 V, can be attributed to the Cu+ oxidation to Cu2+. The signal is typical for copper [10]. The behavior of the Cunanoparticles supported on graphite electrode is similar to the pure copper (Fig. 5 (b) and (c)). Regarding the nanoparticles prepared by the first experiment (Table 1), Fig. 5 (b) shows the

Cyclic voltammogram recorded for pure nickel in 1 M of NaCl at 100 mV/s scan rate shows two characteristic peaks: one anodic peak at +0.42 V for Ni to Ni2+ oxidation and one cathodic peak at +0.37 V for Ni2+ reduction [11] (Fig. 6 (a)). Response of the graphite supported Ni-nanoparticles is very similar to the bulk Ni (Fig. 6 (b) and (c)). The nanoparticles obtained by the experiment 2 (Table 1) shows a pronounced peak in the anodic scan at +0.65 V and a reduction peak in the cathodic scan at +0.15 V. The same anodic and cathodic peaks at +0.7 V and +0.19 V respectively are observed when the low concentrated nickel salt is added to the supporting electrolyte. The difference in the position of the peak potentials between bulk material and nanoparticles is related to the different surface energies and hence adsorption properties of the surfaces.

Cu-Ni nanoparticles (not shown here) reveal a complex behavior through the appearance of “new” peaks, which are probably related to the Cu-Ni surface alloy formation. Further investigations are underway to identify the nature of these new formed kinds of nanoparticles. IV.

CONCLUSION

The application of electrochemical discharges has shown potential results for the fabrication of metallic nanoparticles of different species at large quantities. The method obeys physicochemical mechanisms on which few investigations have been done so far. Alternating the working and counter electrodes’ materials, as well as alternating the presence or absence of additional salts in the electrochemical solutions, is an introduction to the identification of the nanoparticles’ origin. The counter electrode provided the electrochemical system with large quantities of cations by anodic dissolution. Supplemental efforts should be done in this way. ACKNOWLEDGMENT This work was supported by the National Sciences and Engineering Research Council of Canada (NSERC) and Petro Canada. REFERENCES [1]

Figure 6. Cyclic voltammogramms of (a) pure nickel, (b) and (c) graphite supported nanoparticles prepared by electrochemical discharge (experiments 2 and 4 in Table 1, respectively) in 1 M NaCl at 100 mV/s scan rate

C. Discussion The electrochemical discharges in a conductive media play the role of good precursor for the electrochemical reduction of the metallic ions in the solution. This electrochemical reduction is the process by which nanoparticles of these metals are generated. Basically, a metallic ion Mz+ in the solution gains zeto form a metallic nanoparticle of the M-kind. The gas film around the working electrode prevents the eventual eletrodeposition of the formed nanoparticles at the working electrode site. The aim of the conducted experiments is to qualitatively identify the origin of these ions. Ultrafine observations with TEM-EDS and AFM, as well as the electrochemical measurements, have shown that the metallic nanoparticles have two different provenances. When compared to the added salts, the anodic dissolution of the counter electrode has contributed much more significantly to the fabrication of nanoparticles. Voltammetric measurements conducted for all the products synthesized in the present work have confirmed the formation of the metallic nanoparticles as well as their chemical composition. The CVs of the bimetallic

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