Parameters and properties for the preparation of Cu nanocolloids containing polyvinyl alcohol using the electrical spark discharge method

Abstract

Through the use of an electric discharge machine, this study performed the electrical spark discharge method in deionised water under normal temperature and pressure for Cu nanocolloid (CuNC) preparation. The CuNCs had a zeta potential of 12.3 mV, indicating poor suspension stability. The suspension stability was effectively increased (zeta potential 32.5 mV) through the addition of polyvinyl alcohol (PVA) to form PVA-containing CuNCs PVA/CuNCs. Next, the following pulse-width modulation (T_on:T_off) parameters were tested to determine the optimal setting for PVA/CuNC preparation: 10:10, 30:30, 50:50, 70:70 and 90:90 µs. The optimal preparation parameter was then determined according to the absorbance, zeta potential and size distribution results. Finally, the surface properties and crystal structure of the PVA/CuNCs were characterised through transmission electron microscopy (TEM) and X-ray diffraction (XRD). When the T_on:T_off was set to 30:30 µs, preparation efficiency was optimal, as was suspension stability, as indicated by the absorbance value (0.534), zeta potential (32.5 mV) and size distribution (85.47 nm). Transmission electron microscopy revealed that Cu nanoparticles that were more dispersed in the PVA/CuNCs had a diameter smaller than 10 nm and a crystal line width of 0.2028 nm. X-ray diffraction showed that the PVA/CuNCs contained intact Cu crystal structures.

Keywords

Electrical spark discharge method Cu nanoparticles suspension stability parameter control zetasizer

Introduction

Copper (Cu), a common transition metal and the most commonly used metal,¹ has high ductility as well as adequate heat^2–4 and electric conductivity.^5–7 Therefore, Cu has received considerable scholarly attention. In particular, Cu nanoparticle (CuNP) synthesis and application has become a popular topic in scientific and industrial fields. Due to its low resistivity, Cu is often used in cable, electrical and electronic components. In industry, Cu is used in gas sensors, which are crucial to the protection of the environment and human health.⁸ In medicine, CuNP is widely used in antibacterial applications.^9–11 Because nanocolloids’ suspension stability decreases over time, most studies add chemical dispersants^12–15 to lower the pH level¹⁶ or add surfactants.^17,18 In addition, CuNP is used in waste management¹⁹ for environmental protection. Polyvinyl alcohol (PVA) is a widely used water-soluble polymer within the visible spectrum that is nontoxic, nonpollutant, highly transparent and has widespread application in optics, pharmaceutical industry and medicine. It can also be used to enhance nanocolloid stability.²⁰

Cu nanocolloids are prepared using both physical and chemical methods, each with varied applications in different fields. Microwave heating is a fast, simple and low-cost method for the synthesis of nanocomposites. The prepared products have high purity and small particle size distribution, which can be used to prepare CuNPs and related composites.^21,22 Chemical procedures, such as the polyol method,²³ chemical reduction method²⁴ and a novel one-step chemical methods,²⁵ are more commonly used. However, they involve the use of chemical agents that may pose risks to both the environment and human health and are more expensive than physical methods. In efforts to reduce cost and effective produce nanocolloids, this study used the electrical spark discharge method (ESDM),²⁶ an environmentally friendly physical process with high efficiency and short production time. First, a servo control system was used to maintain the electrode gap at a minimal distance.²⁷ A pulse voltage that changed rapidly and periodically was then used to convert electricity to heat through electric spark discharge. After a high-temperature state was achieved, particles were stripped from the electrode surface and subjected to rapid cooling to create nanoparticle (NP). Finally, these particles were dispersed in dielectric fluid to form nanocolloid. Cu wires were used as electrodes, with the upper and lower electrodes as the positive and negative charge, respectively, and 150 mL of deionised water (DW) used as the dielectric fluid. To enhance the suspension stability of the CuNC, 1 mL of 50 ppm PVA was added to the DW. Next, the electric discharge machine (EDM) was run using five different T_on:T_off parameters. After 5 min of discharge, a PVA-containing CuNC (PVA/CuNC) material with adequate suspension stability was produced. Ultraviolet-visible spectroscopy (UV-Vis), Zetasizer, transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterise its properties. PVA/CuNC preparation parameters were found to be suitable when absorbance and zeta potential were at maximum and when size distribution was at minimum.

Material and methods

EDM system

The present study used an EDM for PVA/CuNC preparation. Figure 1 present the discharge processing system. The parameters used included electric current (IP) and T_on:T_off ratio. The voltage output was set to 140 V. The T_on:T_off represents the period of the discharge pulse cycle. By changing the pulse width of the duty cycle, the discharge processing quality can be controlled, whereas changing the T_on:T_off parameters affects the discharge success rate. When the duty cycle was set at 50%, the optimal outcome was achieved. The IP setting represents the current setting; higher IP value indicates higher gap current (I_gap) and faster processing speed, which leads to lower processing precision. This may result in a larger NP size.

Figure 1.

Schematic of the electrical discharge machine.

The ESDM, which is frequently used in precision processing, can also be applied to nanocolloid preparation.^28–30 The principle is to use a minimal electrode distance and the energy generated by high-electricity fields to achieve an electric field intensity greater than the dielectric strength, causing a dielectric fluid breakdown. The passage of electricity through the two circuits generates a spark discharge that converts electricity to heat and generates high temperatures for component melting and for the removal of the component surface materials. Nanocolloid preparation using the ESDM proceeds as follows (Figure 2): pre-electrical discharge; initiation of electrical discharge; ionisation; melting; end of electrical discharge and insulation recovery. As shown in Figure 2(a), Cu wires used as the two electrodes were soaked in the dielectric fluid, and a certain distance was maintained between the electrodes. The voltage is not activated at this point, and the system remains insulated; no gap voltage (V_gap) or I_gap is generated. As shown in Figure 2(b), once the system enters the T_on phase, the servo control system closes the two electrodes to a minimal distance, creating an electric field of sufficient strength to break the insulation of the dielectric fluid. In the process, large amounts of electrons move from the cathode to the anode, forming an electric arc and generating heat. As presented in Figure 2(c), this break in the insulation causes the electrons to affect the neutral atoms in the dielectric fluid, activating them and creating cations. In addition, the rapid entry of electrons into the anode induced the ionisation effect, creating a low-resistance channel and thereby decreasing V_gap and increasing I_gap. As presented in Figure 2(d), continuous electrodeionisation caused cations and anions to constantly affect each other and form an electric arc between the electrodes. Through this process, electricity is converted to an immense amount of heat almost instantly. This melts the copper electrode surfaces, stripping the NPs and causing them to disperse and become suspended in the dielectric fluid. As shown in Figure 2(e), after entering the T_off phase, the incoming voltage supply of the electrodes is cut off. This reduces V_gap and I_gap to zero, returns the dielectric fluid to the insulation state and causes the disappearance of the low-resistance channel. The cooling of the CuNPs in the dielectric fluid marks the completion of the discharge processing cycle. Figure 2(f) shows the entry of the system into the next discharge processing cycle to produce more NPs (before the initiation of the T_on phase).

Figure 2.

Electrical spark discharge process. (a) Before electrical discharge; (b) electrical discharge starts; (c) ionisation; (d) melting; (e) electrical discharge ends and (f) insulation recovery.

Cu nanocolloid and PVA/CuNC preparation through the ESDM

Using the ESDM (which involved using 150 mL of DW as the dielectric fluid), CuNCs were prepared under normal temperature and pressure. When the zeta potential is lower than 30 mV indicates that poor nanocolloid suspension stability and that aggregation will occur easily. As shown in Figure 3, the CuNC zeta potential was 12.3 mV. In other words, its suspension stability was poor. Polyvinyl alcohol was added to improve the dispersibility of the CuNC. Polyvinyl alcohol can adsorb on the colloids and provide a steric effect between the particles, allowing the particles to resist the surface attraction between the particles and achieve a stable dispersion effect in the suspension system.

Figure 3.

Zeta potential of the CuNCs. CuNCs: Cu nanocolloids.

To enhance the suspension stability of the CuNCs, a 50 ppm PVA solution was added to the DW through the following process:

Heat 19 mL of DW and add 1 g of PVA powder to the DW.

Continue heating and use a magnetic stirrer to homogenise the mixture. Once turns transparent, the PVA solution preparation is complete.

Dilute the PVA solution to 50 ppm by adding 1 mL of PVA solution to 4 mL of DW.

Add 1 mL of diluted PVA solution to 150 mL of DW.

Next, upper and lower electrodes composed of Cu wires (99.8% purity) were installed. The servo control system was used to control the distance between the upper and lower electrodes and generate rapidly changing pulse voltages for spark processing. The CuNP powder was evenly distributed in the dielectric liquid through the use of a magnetic stirrer to prevent spark processing failure caused by CuNP aggregation at each end of the electrodes. The optimal T_on:T_off parameter for PVA/CuNC preparation was determined from a total of five (10:10, 30:30, 50:50, 70:70 and 90:90 µs). Table 1 lists the experimental parameters.

Table 1.

Experimental parameters for the preparation of PVA/CuNCs.

Experimental parameter
Anode and cathode	Copper wire (99.8%)
T_on:T_off	10:10, 30:30, 50:50, 70:70 and 90:90 (µs)
Discharge time	5 min
Dielectric fluid	DW: 150 mL PVA: 1 mL (50 ppm)
Current setting (IP)	3
Voltage	140 V
Temperature	25°C
Atmospheric pressure	1 atm

PVA: polyvinyl alcohol; DW: deionised water; CuNCs: Cu nanocolloids.

Analysis of PVA/CuNC characteristics

The optical properties, suspension stability, NP size, crystal structure and surface properties of these PVA/CuNCs were examined through UV-Vis, Zetasizer, XRD and TEM. Ultraviolet-visible spectroscopy measures absorbance, which is used to compare gel concentration and determine the discharge success rate and characteristic wavelengths. Zetasizer measures the zeta potential and NP size distribution. When the absolute value of the zeta potential exceeds 30 mV, this indicates that the nanocolloid has adequate suspension stability. Size distribution involves the assessment of the NPs’ size and distribution. X-ray diffraction was specifically used to analyse the diffraction peaks of the PVA/CuNCs to confirm that the structure contains no other derivatives and ascertain that the inclusion of PVA did not affect the original structure of the CuNPs. The form and distribution of CuNPs in the PVA/CuNCs were examined through TEM.

Results and discussion

The UV-Vis results revealed that the characteristic wavelength of each PVA/CuNC was 261 nm (Figure 4). Higher absorbance indicates a higher CuNP preparation efficiency. Absorbance is positively correlated with concentration; therefore, the UV-Vis results were used to discuss the discharge success rate. Table 2 compiles the results of UV-Vis of the CuNCs. The PVA/CuNCs prepared when T_on:T_off = 10:10 µs had an absorbance value of 0.462. Those prepared when T_on:T_off = 30:30, 50:50, 70:70 and 90:90 µs had absorbance values of 0.534, 0.406, 0.424 and 0.397, respectively. The PVA/CuNCs had the highest absorbance value and concentration when the T_on:T_off was set to 30:30 µs. Regarding optical properties, PVA/CuNC preparation was the most efficient when this parameter was used.

Figure 4.

Results of UV-Vis of the PVA/CuNCs. PVA: polyvinyl alcohol; UV-Vis: ultraviolet-visible spectroscopy; CuNCs: Cu nanocolloids.

Table 2.

Absorbance of PVA/CuNCs prepared using five sets of T_on:T_off parameters.

T_on:T_off (µs)	Absorbance
10:10	0.462
30:30	0.534
50:50	0.406
70:70	0.424
90:90	0.397

PVA: polyvinyl alcohol; CuNCs: Cu nanocolloids.

Table 3 shows the suspension stability and size distribution of the PVA/CuNCs. The zeta potential of each PVA/CuNC exceeded 30 mV; in other words, they exhibited adequate suspension stability. The size distribution of each PVA/CuNC was smaller than 100 nm, indicating that each particle size met the NP. These results were used to identify parameters more suitable for PVA/CuNC preparation. As shown in Figure 5(a) and (b), the zeta potential and size distribution of the PVA/CuNCs were the highest and smallest (32.5 mV and 85.47 nm), respectively, when the T_on:T_off was set to 30:30 µs. In essence, those PVA/CuNCs had optimal suspension stability and rarely exhibited CuNP aggregation. To reiterate, this means that 30:30 µs was the optimal parameter for PVA/CuNC preparation.

Table 3.

Results of Zetasizer testing on PVA/CuNCs using five sets of T_on:T_off parameters.

T_on:T_off (µs)	Zeta potential (mV)	Size distribution (d.nm)
10:10	31	86.23
30:30	32.5	85.47
50:50	31.1	91.77
70:70	32	89.53
90:90	32.1	87.69

PVA: polyvinyl alcohol; CuNCs: Cu nanocolloids.

Figure 5.

Zetasizer of PVA/CuNCs (30:30 µs): (a) zeta potential and (b) size distribution. PVA: polyvinyl alcohol; CuNCs: Cu nanocolloids.

Figure 6 depicts the TEM images of the PVA/CuNCs prepared when T_on:T_off = 30:30 µs. Figure 6(a) presents the testing results at 40,000 × magnification on a 100 nm scale. The darker sections represent the CuNPs, with those on the left being more dispersed and those on the right having larger particle diameter. This variation is due to CuNP aggregations that were in turn caused by poor discharge efficiency. Figure 6(b) provides a closeup view of the white rectangle in Figure 6(a) at 100,000 × magnification and on a 20 nm scale. Cu nanoparticle aggregation was observed, but some of the particles retained their round shape. Most of them were 10 nm in diameter. Figure 6(c) presents a closeup view of the white rectangle in Figure 6(b) at 1,000,000 × magnification, on a 2 nm scale. The black sections represent the CuNPs, the crystal lines of which are clearly observable. The crystal orientation was (111). The crystal line width of 0.2028 nm corresponds to ICDD 00-004-0836 in the XRD database.

Figure 6.

TEM images of PVA/CuNCs (30:30 µs): (a) scale 100 nm (b) scale 20 nm (c) scale 2 nm. PVA: polyvinyl alcohol; TEM: transmission electron microscopy; CuNCs: Cu nanocolloids.

Figure 7 depicts the results of XRD of the PVA/CuNCs that were prepared when T_on:T_off = 30:30 µs. The three diffraction peaks of Cu that can be clearly seen correspond to ICDD 00-004-0836 in the XRD database. In sequential order, these peaks had Bragg reflection angles of 43°, 50° and 74° and crystal orientations of (111), (200) and (220). Also observable are the two diffraction peaks of CuO, which correspond to ICDD 00-048-1548 in the XRD database and, in sequential order, had Bragg reflection angles of 35° and 38° and crystal orientations of (−111) and (111). Subsequently, the diffraction peak-related data of this result was substituted into the Scherrer formula; the calculated average crystallite size of PVA/CuNP was 24.012 nm. In sum, the PVA/CuNCs prepared using the ESDM contained only the crystallised structures of Cu and CuO; no other derivatives were present. This confirms that the PVA solution did not affect Cu structure and that the ESDM is an environmentally friendly procedure.

Figure 7.

XRD patterns of PVA/CuNCs (30:30 µs). PVA: polyvinyl alcohol; XRD: X-ray diffraction; CuNCs: Cu nanocolloids.

Conclusions

The ESDM was used to prepare the PVA/CuNCs. First, 1 mL of 50 ppm PVA solution was added to 150 mL of DW to improve its suspension stability. Subsequently, UV-Vis and Zetasizer were used to determine the optimal T_on:T_off parameter for PVA/CuNC preparation from among 10:10, 30:30, 50:50, 70:70 and 90:90 µs. Finally, the properties of the PVA/CuNCs were characterised through TEM and XRD. The main conclusions are summarised as follows:

The present PVA/CuNC preparation process, which is conducted under normal temperature and pressure using the ESDM, does not cause environmental pollution and is less harmful to human health compared with other methods. It is also rapid to perform.

The CuNCs had a zeta potential of 12.3 mV which, being lower than 30 mV, indicated poor suspension stability. By comparison, the maximum zeta potential of the PVA/CuNCs was 32.5 mV, considerably higher than that of the CuNCs. This suggests that addition of the PVA solution substantially increased the suspension stability of the nanocolloid.

Ultraviolet-visible spectroscopy revealed the appearance of a substantial absorbance peak at 261 nm, a result that is consistent with the optical properties of CuNP. When the T_on:T_off was set to 30:30 μs, the PVA/CuNCs had a zeta potential of 32.5 mV, an optimal absorbance value of 0.534 and the smallest size distribution of 85.47 nm. Also, suspension stability was optimal under this setting. Therefore, it constitutes the most suitable parameter setting for CuNC preparation.

The round shape of the CuNPs in the PVA/CuNC was observed through TEM. The particle diameter of the unaggregated CuNP particles was less than 10 nm. The line width and orientation of the crystals were 0.2028 nm and (111), respectively. These results are in line with the properties of ICDD 00-004-0836 listed in the XRD ICDD database.

The crystal structures of Cu and CuO were identified through XRD. Cu had three diffraction peaks that in sequential order had Bragg reflection angles of 43°, 50° and 74° and crystal orientations of (111), (200) and (220). CuO had two diffraction peaks that in sequential order had Bragg reflection angles of 35° and 38° and crystal orientations of (−111) and (111). These results indicate that no other derivatives were generated in the PVA/CuNC preparation process, and that the PVA solution did not affect the crystal structure of Cu.

Footnotes

Acknowledgments

The authors would like to thank the Precision Research and Analysis Center, National Taipei University of Technology for technically supporting this research.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to thank the Ministry of Science and Technology (MOST 109-2221-E-027-063-) for financial support of this research.

ORCID iD

Kuo-Hsiung Tseng

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