Green synthesis of copper nanoparticles with ultrasound assistance

1 Introduction

In recent years, there has been continual growth availability of technological devices, which has led to a rise in the amount of associated waste products or e-wastes. E-wastes can be detrimental to the natural environment, as they can contain toxic and polluting elements. Furthermore, they contain non-negligible amounts of precious and valuable metals, such as gold, silver, and copper (Cu) [1]. A satisfactory method by which to correctly dispose of e-wastes, specifically how to recycle them, is still a matter of study, and only a small fraction of e-wastes can be treated to recover precious and valuable metals. Hydrometallurgy and pyrometallurgy are two possible techniques that can be used to treat e-wastes are. Even though pyrometallurgical processes are the most widely implemented in the industry, hydrometallurgical processes have some advantages, such as higher selectivity and lower gas emissions. However, to mitigate the environmental impact due to metal recovery from e-wastes via hydrometallurgy, the process must use green reagents and techniques. Since the publication of the work of Paul Anastas and John C. Warner, finding ways to minimize the environmental impact, where possible, is considered an urgent topic in the field of chemical research [2].

In the presented work, Cu nanoparticles have been synthesized using the leaching solution that is used to treat end-of-life printed circuit boards. In addition to the intrinsic value of Cu, this value can be enhanced further by recovering it in the form of nanoparticles, which exhibit promising properties relative to the properties of the bulk material [3]. Cu nanoparticles have been synthesized using a green technique, which involves green reagents and ultrasound [4].

Interest in Cu nanoparticles has increased recently due to their application in catalysis [5], thermal dissipation fluids, optical and magnetic devices, metal injection molding, flexible electronics, and as antifungal and antimicrobial agents [6]. Several methods have been suggested to produce them [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. In particular, physical methods can be used, such as pulsed laser ablation, vacuum vapor deposition, pulsed wire discharge, thermal decomposition, and mechanical milling. Alternative chemical methods include chemical reduction, microemulsion techniques, sonochemical reduction, electrochemical, photoirradiation, microwave assisted, and hydrothermal synthesis [24], [25].

As far as chemical reduction concerns, Cu salt can be reduced by a reducing agent, such as sodium borohydride, hydrazine, ascorbate, polyol, isopropyl alcohol with cetyltrimethylammonium bromide (CTAB), or glucose [26]. The most widely used capping agent is polyvinylpyrrolidone, although oleic acid, carboxylic acids (glycolic acid, lactic acid, acetic acid, etc.) poly(allylamine), and polyethylene glycol are amongst other capping agents that are commonly employed [27].

The green credentials of ultrasound technology are widely accepted in chemistry research [28]. Its physical and chemical effects derive from the cavitational collapse that produces locally extreme conditions that promote the formation of reactive chemical species, which cannot be easily obtained under conventional conditions.

2 Materials and methods

Pure Cu recovered from the leaching solution of the pre-treatment of printed circuit boards was used the raw material in this study. Cu was extracted from end-of-life mobile phones, as described in Figure 1. The electronic scraps were pre-treated with a nitric acid solution to remove Cu, silver, tin and iron. The composition of the solution is shown in Table 1. The Cu was removed from the solution by cementation with iron (Equation 1).

Figure 1:

Scheme of the process.

The Cu powder was separated by filtration and then washed three times with deionized water. All reagents were purchased from Sigma-Aldrich, St. Louis, USA.

After the reaction, 0.0263 g of the Cu powder was dissolved in 100 μl of nitric acid, after which 40 ml of water was added to the solution. The acid solution was neutralized with a 10 ml solution 0.6 m of sodium hydroxide. Two different reducing agents were tested: L-acid ascorbic (8 g/l) and sodium borohydride (0.1 g/l).

The effects of ultrasound on the sizes of the nanoparticles, on the reaction time, and on the yield were investigated. The ultrasound irradiation was provided by a VS 70T probe (13 mm diameter in titanium alloy Ti6Al4V), which was immersed in the solution, and linked to the ultrasound generator Sonopuls HD 2070 sonicator of Bandelin, Berlin, Germany. This sonicator is capable of supplying ultrasonic power of 75 W/cm² at 20 kHz. The experiments were carried out with an amplitude of 85%. The powder obtained was characterized by a Sterescan 440 scanning electon microscope (SEM) (Cambridge Instruments, Cambridge, UK) equipped with a PV9800 (Philips, Amsterdam, Netherlands) energy-dispersive X-ray spectroscopy (EDS) detector, by X-ray diffraction (XRD), using a D500 diffractometer (Siemens, Berlin, Germany) with nickel-filtered CuKα (λ=1.5405 Å), at 40 kV and 30 mA. The size and the structure of the powder were characterized by transmission electron microscope (TEM) with a JEOL 200CX. For TEM observation, the powder was dispersed in isopropyl alcohol and sonicated for 1 min. The particle size distribution was analyzed by laser light scattering with a Mastersizer (Malvern, Malvern, UK). UV spectroscopy was performed with a (Jasco, Easton, USA) Model V-530 Spectrophotometer. Solutions were analyzed by genesis inductively coupled plasma (ICP) spectrometer (Spectro, Kleve, Germany).

3 Results and discussion

The powder recovered from the nitric acid solution by cementation with Fe was analyzed by SEM, EDS, and ICP. Figure 2 shows the SEM image of the Cu powder and the corresponding EDS spectrum. The powder was constituted by an agglomeration of particles of about 1 μm. From EDS analysis, the powder was found to be composed of Cu without the presence of other elements. The ICP analysis confirmed the purity of the Cu. In order to achieve the synthesis of Cu nanoparticles, the powder was used to prepare a solution containing Cu salt, as described in Section 2. The powder was used directly in the reaction.

Figure 2:

SEM image and EDS spectrum of the precipitated Cu powder.

3.1 L-ascorbic acid

When using L-ascorbic acid, ultrasound assistance was found to reduce the reaction time, as shown in Figure 3. The Cu reduction with ultrasound assistance was completed within 10 min, whereas the process required approximately 17 h with conventional heating at 80°C [24].

The UV-vis absorption spectrum of the solution treated under ultrasound for 5 min had a peak at 580 nm, confirming that the Cu nanoparticles were produced (Figure 4A), in agreement with the literature [29], [30]. After 10 min, the peak became more pronounced, suggesting that the complete reaction occurred after this time.

Figure 3:

Solution with reduced Cu after 10 min of sonication using L-ascorbic acid as a reducing agent.

The XRD analysis performed on the nanoparticles obtained after a reaction time of 10 min is shown in Figure 4B. The peaks were attributed to the Cu (ICSD n. 98-005-3247). Meanwhile, SEM images of the powders obtained with and without ultrasound are shown in Figure 5. As can be seen, the mean size of the particles produced without ultrasound is shown to be approximately 1 μm (Figure 5B), whereas the particles produced with ultrasound were smaller than 100 nm (Figure 5A).

Figure 4:

UV-vis absorption (A) and XRD (B) spectra of the Cu nanoparticles solution synthesized with L-ascorbic acid and ultrasound irradiation for 10 min.

Using laser diffraction analysis, we find that the dimensions of the nanoparticles produced using L-ascorbic acid with ultrasound irradiation were approximately 5 nm (Figure 6). TEM analysis showed that nanoparticles with diameter of about 5–7 nm were present (Figure 6). Moreover, we verified that ultrasound increased the yield of the reaction. In particular, two more concentrated solutions containing 20 g/l Cu nitrate and 40 g/l of L-ascorbic acid, respectively, without any capping agent, were prepared. One was heated for 1 h at 80°C, whereas the other one was treated with ultrasound for 10 min. In the first case, using conventional heating, 0.01499 g of Cu powder precipitated, whereas in the second case, using ultrasound irradiation, 0.07608 g of Cu powder precipitated. Therefore, the yield obtained with ultrasound assistance was approximately 5 times higher, but took less time to produce (Figure 7).

Figure 5:

SEM images of powder obtained using L-ascorbic acid (A) without ultrasound and (B) with ultrasound.

Figure 6:

Malvern analysis of the powder with (A) ultrasound and (B) TEM bright field image of the powder obtained with ultrasound, using L-ascorbic acid as reducing agent.

Figure 7:

Comparison between the reaction carried out (A) with ultrasound and (B) without ultrasound, using L-ascorbic acid.

3.2 Sodium borohydride

For comparison, the synthesis of Cu nanoparticles was also performed with sodium borohydride, both without and with the assistance of ultrasound. With this reducing agent the reaction occurred within a shorter time relative to L-ascorbic acid, and heating was not required in the reaction without ultrasound. However, we observed that nanoparticles dissolved after 6 h, whereas the nanoparticles produced with L-ascorbic acid remained stable for weeks.

The UV spectroscopy analysis confirmed that Cu nanoparticles were produced, and the characteristic peak at 580 was observed. Meanwhile, SEM images of the powders obtained with and without ultrasound are shown in Figure 8, where agglomerates of nanoparticles smaller than 300 nm are visible. The Malvern analysis showed that nanoparticles obtained with ultrasound had a particle size of approximately 6 nm, whereas the nanoparticles produced without ultrasound were characterized by a mean size of about 15 nm (Figure 9). The analysis carried out with TEM showed that the nanoparticles produced with the assistance of ultrasound had a size of about 7 nm, whereas the ones produced without ultrasound were larger, with sizes ranging between 10 and 20 nm, thus confirming the results of the Malvern analysis (Figure 10).

Figure 8:

SEM images of the nanoparticles obtained (A) with ultrasound and (B) without ultrasound, using NaBH₄ as a reducing agent.

Figure 9:

Malvern analysis of the powder using NaBH₄ as reducing agent (A) with ultrasound and (B) without ultrasound.

Figure 10:

TEM bright field images of the powder obtained (A) with ultrasound and (B) without ultrasound, using L-ascorbic acid as a reducing agent.

4 Conclusions

In this work, a method has been developed to utilize Cu recovered from electronic scrap as a raw material for the production of Cu nanoparticles. The use of L-ascorbic acid as a reducing agent was investigated in a water solution, which was found to be effective in reducing Cu. The use of ultrasound proved to be critical for increasing the production of the nanoparticles. Specifically, nanoparticles with a mean size of about 5 nm were obtained with ultrasound, whereas without ultrasound the reaction took several hours and the final powder produced had a mean size of 1 μm. Furthermore, the use of L-ascorbic acid as a reducing agent with ultrasound allowed for the production of Cu nanoparticles with a size that can be comparable to nanoparticles produced with sodium borohydride.