# Screen-printed electrodes on paper using copper nano- and micro-particles

Anesu Nyabadza, Anouk Plouze, Saeid Heidarinassab, Mercedes Vazquez, Dermot Brabazon

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Anesu Nyabadza, Anouk Plouze, Saeid Heidarinassab, Mercedes Vazquez, Dermot Brabazon. Screen-printed electrodes on paper using copper nano- and micro-particles. Journal of Materials Research and Technology, 2024, 29, pp.5189-5197. 10.1016/j.jmrt.2024.03.016. hal-05319377

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Submitted on 17 Oct 2025

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# Screen-printed electrodes on paper using copper nano- and micro-particles

Anesu Nyabadza<sup>a, b, c, *</sup>, Anouk Plouze<sup>c, d</sup>, Saeid Heidarinassab<sup>a, b, c</sup>, Mercedes Vazquez<sup>a, b, c</sup>, Dermot Brabazon<sup>a, b, c</sup>

## ARTICLE INFO

### Keywords:

Screen-printing

Battery electrodes

Copper electrodes

Printed batteries

Nanoparticles

Additive manufacturing

## ABSTRACT

Screen-printed copper electrodes have the potential to replace silver and lithium electrodes in printed batteries and sensors owing to the abundance of copper, its competitive electrical properties, and the low cost of raw materials. Herein, copper microparticle-glycerol-based inks were screen-printed onto paper to develop highly conductive electrodes. The conductivity and particle connectivity enhancement of screen-printed electrodes are the main research topics. These parameters translate to usability including increased sensitivity for sensor applications and reduced resistivity for battery electrode applications. CuO nanoparticles of 50 nm were deposited onto the electrodes at a concentration of 1 µl/cm², which led to a 4-fold decrease in sheet resistivity from 200 to 50 Ω/m². The increased connectivity and formation of new channels due to the addition of nanoparticles were visualised via field emission scanning electron microscopy. The CuO nanoparticles were developed via a hybrid approach incorporating wet chemistry for high yield and laser ablation for colloidal conductivity and Zeta potential enhancement which reached up to –45 mV. An optimal screen-printing viscosity of 62 mPa s was achieved through a precise 1.95:1 mass ratio of Cu microparticles to glycerol. Heat treatment at 100 °C after screen-printing enhanced particle connectivity by reducing glycerol content in the ink.

## 1. Introduction

Screen-printed electrodes serve as versatile platforms for the development of sensing devices, printed circuits, and printed batteries [1]. Screen-printing can be conducted on various substrates including paper, ceramics, metals, polymers, and glass [2]. Screen-printing offers the flexibility to produce diverse cell or electrode patterns, encompassing single electrodes, traditional three-or-more electrode cells, interdigitated electrodes, flow cells, and more, catering to a wide array of applications. The ease of preparation and cost-effective production enables the mass customization of electrode configurations, encompassing different substrates, geometries, shapes, and sizes. Beyond their primary advantage of customizable manufacturing, screen-printed electrodes provide the capability to create tailored surfaces for specific analyte detection applications, clinical settings, and academic research. While screen-printing may have lower dimensional precision and flexibility in printed shapes (which are limited by the stencil) compared to techniques like aerosol jetting or high-resolution inkjet printing, it compensates

with higher scalability, faster production speeds, and ease of operation on a mass production scale. Furthermore, the dimensional accuracy of screen-printing is generally acceptable for most applications, contributing to its widespread use.

Additionally, screen-printing finds utility in the fabrication of printable batteries, with lithium being a prevalent material due to its desirable properties, including high energy density, lightweight characteristics, high voltage, low self-discharge, long cycle life, rechargeability, and extensive literature availability [3]. However, the increasing demand for batteries, coupled with the depletion of lithium resources, necessitates exploration into alternative materials such as Cu [4]. Copper, abundant and highly conductive, emerges as a promising candidate for both anode and electrolyte applications in batteries, offering advantages like thermal and chemical stability, high theoretical specific capacity, affordability, and environmental friendliness. The integration of nanomaterials further enhances battery electrode performance by increasing energy density and efficiency [5]. Nanoparticles contribute to improved surface area for ion transfer, enhanced electrochemical

<footnote><sup>a</sup> I-Form Advanced Manufacturing Centre Research, Dublin City University, Glasnevin, Dublin, 9, Ireland</footnote>

<footnote><sup>b</sup> EPSRC & SFI Centre for Doctoral Training (CDT) in Advanced Metallic Systems, School of Mechanical & Manufacturing Engineering, And School of Chemical Sciences, Dublin City University, Glasnevin, Dublin, 9, Ireland</footnote>

<footnote><sup>c</sup> Advanced Processing Technology Research Centre, Dublin City University, Glasnevin, Dublin, 9, Ireland</footnote>

<footnote><sup>d</sup> Conservatoire national des arts et métiers (CNAM), 61 Rue Du Landy, 93210, Saint-Denis, France</footnote>

<footnote>* Corresponding author. I-Form Advanced Manufacturing Centre Research, Dublin City University, Glasnevin, Dublin, 9, Ireland. E-mail address: anesu.nyabadza@dcu.ie (A. Nyabadza).</footnote>
performance, higher capacity, and enhanced cycling stability. Moreover, NPs can be designed to be compatible with electrolytes, reducing the risk of side reactions or degradation.

Screen-printing is the process of transferring a stenciled design onto a surface using a mesh screen, ink, and a squeegee. The stencil is placed between the substrate and the ink. The squeegee spreads the ink over the surface of the stencil, allowing the design to be printed onto the substrate through the mesh which has holes that enable particles or inks to pass through when enough pressure is applied (via the squeegee).

The modification of commercial screen-printed electrode surfaces typically employs three established methods: (1) ink mixing with modifying agents before curing, (2) electrochemical deposition of metallic precursors, or (3) drop-casting of nanoparticles [6]. The first method carried out before ink curing, demands meticulous control of parameters like curing temperature and mixing recipes to ensure batch reproducibility. The latter two methods, performed post-electrode preparation, are better suited for working with commercially available screen-printed electrodes. For example, Hughes et al. utilised the third method during which they dip-coated commercially available screen-printed carbon electrodes into gold nanocolloid for enhanced DNA detection [6]. Researchers are depositing nanomaterials onto screen-printed electrodes to achieve the required functionality (e.g., sensing capability or increased electrical conductivity). Ambaye et al. screen-printed carbon electrodes using a carbon paste to produce an electrochemical sensor that can detect nitrates in wastewater [7]. The screen-printed electrode was functionalized by drop-casting 6 µL of Cu nanoparticles onto the sensing electrode. Inam et al. conducted similar research utilizing screen-printed electrodes for nitrate detection [8]. The main result was that the addition of Cu nanoparticles onto the bare Ag screen-printed electrode caused a 77.4% increase in the electroactive surface area, leading to a nitrate sensing range between 0.05 and 5.0 mM. Similarly, Thakur et al. produced a screen-printed sensor for sulfide quantification in the presence of interferents such as nitrite, sulfite, citrate, acetate, urea, and thiosulfate [9]. To create the sensor, a commercial carbon-graphite ink was screen-printed onto a polyester flexible film, followed by printing the reference electrode using a commercially available silver/silver chloride paste. A commercially available dielectric paste was screen-printed to cover the connection and define the carbon-graphite working electrode. Finally, a commercially available Cu nanoparticle ink was printed over the carbon-graphite working electrode, resulting in the typical 3-electrode setup. This produced robust and disposable sensing platforms that are low-cost and can be produced in masses owing to screen-printing technology. The addition of Cu nanoparticles resulted in increased sensitivity and conductivity owing to the high reactivity and conductivity of Cu nanoparticles.

Gacia et al. screen-printed Ag and Ag-Zn electrodes onto paper substrates to produce paper-based batteries [10] demonstrating the ability of screen-printing to use low-cost materials such as paper to produce flexible batteries. Furthermore, Ostfeld et al. printed inductors, capacitors, and resistors on flexible polymers [11]. Hayat et al. reviewed disposable screen-printed electrodes for environmental monitoring applications in 2014 [12]. The research team concluded that nanomaterials are required to increase the sensitivity and efficiency of commercially available screen-printed electrodes to advance the field. In most articles, the ink formulation process is an imperative part that requires optimization to match the printer's acceptable viscosity.

In this study, copper electrodes are created through screen-printing using Cu microparticle-based inks and subsequently coated with Cu NPs. Paper is chosen as the substrate material, offering advantages over polymers, including enhanced flexibility, superior ink-soaking capacity, lower cost, and excellent adhesion properties, eliminating the need for heating [10]. Notably, Cu NPs are produced for the first time using a hybrid approach that combines wet chemistry with Pulsed Laser Ablation in Liquid (PLAL). This method enables the customization of electrical conductivity and Zeta potentials by adjusting laser processing parameters. The incorporation of NPs after screen-printing is known to

enhance electrode connectivity and conductivity, crucial properties for both sensing and battery electrodes. This article introduces the optimization of material properties such as viscosity, conductivity, mass ratios, and energy (laser power) in the production process, suggesting a potential for mass production of these electrodes. Furthermore, the materials employed in this study including copper powder, glycerol, and paper, are cost-effective and abundant in comparison to conventional ones such as silver, lithium, and polymer substrates [10], aligning with global sustainability goals. The connectivity of particles within the electrodes was analyzed via field emission scanning electron microscopy (FESEM) and Keyence optical microscopy while the chemical composition was examined via Energy-dispersive X-ray spectroscopy (EDX) and Fourier-transform infrared spectroscopy (FTIR). There is a lack of analysis in the literature on the underlying mechanisms for optimizing screen-printed copper electrodes such as viscosity optimization and nanoparticle enhancement. This study aims to address this literature gap by investigating the effect of nanoparticles on microparticle connectivity, conductivity, and chemical composition within screen-printed inks.

## 2. Materials and methods

### 2.1. Screen-printing of copper electrodes

Copper electrodes that have potential applications in battery electrodes or electrochemical sensing applications were screen-printed onto paper substrates (A4 White Paper, 500 sheets, 75 gsm). Copper microparticle inks were formulated by mixing Cu powder (Sigma Aldrich, Ireland Limited, spheroidal powder, 10–25 µm, 98% purity) with glycerol (Sigma Aldrich, 99.9%) to form an ink with a specific viscosity acceptable for high-resolution screen-printing (DEK Horizon APiX Screen Printer). The ink formulation required experiments to optimize the ink viscosity such that the print quality was optimized. The viscosity of the ink depends on the powder-to-glycerol mass ratio. Several combinations were trialed to produce good-quality prints. The best Cu powder-glycerol mass ratio was found at 1.95:1 and the viscosity and shear stress measurements were performed on a Brookfield DVNext Cone/Plate Viscometer at 25 °C.

The quality of the print was assessed via visual inspection and dimensional analysis with the aid of optical and electron microscopes. The demonstrator electrode was a 3-pin electrode. This design has sufficient complexity to judge the quality of the print. It includes 3 electrodes close to each other which can be used to check whether or not the ink is too fluid and it also has vertical and horizontal gaps between electrodes to assess dimensional accuracy. The size of the electrode is also within the size of the battery electrode in printed electronics.

After screen-printing, some electrodes underwent a coating procedure where Cu NPs were coated onto the electrodes in pursuit of increasing the conductivity of the electrodes via the reduction filling of micro gaps between the printed microparticles, an increase in surface area of electrical charge carriers, and high electrical conductivity (high surface-area-to-charge ratio and availability of free electrons) of the Cu NPs.

### 2.2. Copper nanoparticle fabrication

A hybrid NP fabrication approach was employed involving a wet chemistry technique followed by a laser ablation process. A schematic of the synthesis process is shown in Fig. 1. 30 g of Cu powder (spheroids, 10–25 µm, 99% purity, Sigma Aldrich, Ireland) was added to a glass beaker and 50 ml of concentrated ammonium chloride solution (ammonium chloride buffer solution pH 11, Sigma Aldrich, Ireland Limited, 50 ml) was added and mixed with a spatula. This ratio of Cu powder to ammonium solution enables the Cu powder to be in excess at reaction completion. The timer was started and the reaction reached completion at 100 min. The beaker contents were mixed every 10 min with a spatula. The resulting colloid was collected, leaving the unreacted
<figure><img src="image_4.png" /><figcaption>Fig. 1. Nanoparticle hybrid fabrication process using the wet-chemical production process as feedstock for the PLAL process.</figcaption></figure>

or excess copper powder on the bottom of the reaction vessel. 36 samples (each 1 ml) were extracted from the resulting 50 ml of Cu colloid. Each of the 36 samples underwent PLAL at various ablation parameters to investigate the effects thereof.

A 3 × 3 full factorial design of experiments was implemented to investigate the effect of the ablation parameters on the stability (measured via Zeta potentials) and conductivity of the colloid. A Nd: YAG 1064 nm laser (10 kHz repetition rate and 0.6 ns pulse width, Bright solutions) was used for ablation processes. The laser processing parameters investigated were scan speed (10, 100, 1000, and 3000 mm/ s), power (1.3, 1.4, and 1.5 W), and height at which the laser beam is focused within the ablation medium (5, 8, and 12 mm from the bottom of the ablation vessel). Small vials (3 mm diameter and 15 mm height) were used during the ablation process. These enable the ablation time to be reduced (smaller volume of colloid) and a smaller ablation area. The ablation time was kept constant at 4 min.

### 2.3. Characterisation

The Cu colloid was characterized soon after the wet chemistry step and also soon after the PLAL step. FTIR (PerkinElmer Spectrum Two FTIR) was employed for optical property investigations while dynamic light scattering was employed for Zeta potential and colloidal conductivity measurements (Malvern Zetasizer Ultra). Field emission scanning electron microscopy (FESEM-Hitachi S5500 Field Emission SEM) and EDX were employed for morphology and chemical composition analysis of the printed electrodes. A piece of the electrode was cut using standard scissors and placed on a gold SEM grid from FESEM and EDX analysis. An electrode composed of copper microparticles and one with both Cu microparticles and NPs were analyzed via FESEM and EDX. 4-Point probe (Sherescan instrument Version 4.03 MRN-111-EN, Mechatronics Manufacturing B.V.) was used for sheet resistivity measurements (an average of 100 readings per cm² was used in the results). The resistivity of electrodes was also calculated by Equation (1). The resistance (R) was measured via a multimeter, and the length (L) and width of the area of interest within the electrode were measured via Vernier calipers. The thickness was measured via microscopy.

$$p = \frac{RA}{L} \qquad (Equation 1)$$

where p is the resistivity (Ωm), R is the resistance (Ω), A is the area (width by thickness in m²) and L is the length (m). The length, width, and thickness were measured from the bottom of the middle electrode (sensing electrode) for all experiments and were kept constant at 5 mm, 3 mm, and 50 µm respectively. The multimeter was connected at the ends of the area of interest for each measurement, 5 recordings were

taken after stability, and mean results were attained to generate a 95% confidence interval about the mean.

## 3. Results

### 3.1. Screen-printed electrodes

Fig. 2a displays the screen-printed electrodes examined in this study. The optimal viscosity, crucial for achieving desirable results, was determined to be 62 mPa s. This viscosity was attained by maintaining a 1.95:1 mass ratio of Cu powder to glycerol. This specific ratio demonstrated superior particle attachment and enhanced printability of the inks on paper substrates, as evaluated through Keyence optical microscope analysis and visual inspections post-printing. Fig. 2b highlights instances of unsuccessful prints, where partial printing occurred due to excessively high viscosity in certain ink formulations. As anticipated, an escalation in powder mass resulted in increased viscosity. The introduction of microparticles in the fluid amplified friction, impeding fluid movement and elevating fluid density. It is worth noting that microparticles induce a non-Newtonian behavior in the fluid, a phenomenon leveraged by researchers in micro/nanofluids for applications such as heating and cooling. Fig. 2c provides a glimpse of the optimized printed electrodes, showcasing the successful implementation of the identified optimal conditions.

Following the printing process, some electrodes underwent heat treatment with the aim of reducing glycerol content and enhancing particle connectivity. The augmentation of connectivity occurs through the thermal-induced motion of glycerol molecules. The heat treatment temperature was consistently maintained at 100°, while the holding time varied between 30 min and 6 h. Subsequent to heat treatment, the electrodes underwent analysis via FTIR both before and after the process. FTIR effectively illustrates the organic bonds originating from glycerol (C₃H₈O₃), with transmittance percentage directly proportional to the glycerol concentration within the electrode. Fig. 3a provides the FTIR results, demonstrating that a 6-h heat treatment nearly eradicated glycerol, evident in the absence of glycerol transmittance bands. However, this extended treatment duration was deemed excessive, as it resulted in particle detachment from the paper substrate due to the absence of a binding agent. Microscopy images in Fig. 3b depict poorly attached electrodes, showcasing visible gaps and heterogeneity within the printed electrode structure. A 1-h heat treatment also effectively reduced glycerol concentration but was deemed too excessive. Notably, a 30-min heat treatment produced improved particle attachment results while still reducing glycerol content, as indicated by an increase in transmittance (indicating a decrease in concentration). This shorter heat treatment duration also mitigated micro-gaps within the printed
<figure><img src="image_5.png" /><figcaption>Fig. 2. a) Image of mesh design used for screen-printing; b-c) screen-printed electrodes and; d) copper nanocolloids.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. a) FTIR spectra of copper electrodes after heat treatment with increasing holding times; b-c) light microscopy images of copper electrodes before and after heat treatment respectively.</figcaption></figure>

electrodes, as illustrated in Fig. 3c and d, depicting the microstructure before and after heat treatment, respectively. The transmittance bands around 3000 $cm^{-1}$ are attributed to C–H stretching within glycerol molecules [13–15]. The bands between 1300 and 1200 $cm^{-1}$ and those between 1100 and 1000 $cm^{-1}$ are attributed to C–O stretching and CO–O–CO bonds respectively [14].

## 4. Nanoparticle enhancement

The relationships between the outputs, namely Zeta potential and conductivity, and the inputs (power, speed, and height of laser focus) are non-linear in nature, a characteristic commonly observed in PLAL. Fig. 4 presents 3D surface plots illustrating the intricate interplay between the

laser focus height and scan speed on the absolute (Abs) Zeta potential of colloids. The Zeta potential, serves as an indicator of colloid stability, with values exceeding 30 mV indicating stability [16–18]. Post wet chemistry, the stability was measured –18 mV, suggesting negative charges and non-stability, foreshadowing potential particle agglomeration and erosion. In just 4 min, PLAL significantly increased the absolute Zeta potential to 45 mV. Notably, certain PLAL conditions exacerbated stability issues, underscoring the imperative need for optimization. Table 1 provides valuable data for machine learning scientists to discern patterns and generate predictive and optimization models, a scarcity in current PLAL literature. Surface plots reveal that laser power minimally impacted stability, enabling the use of the lowest laser power (1.3 W) to achieve requisite stability while simultaneously reducing power costs.
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Response surface of the effect of process input parameters on the absolute Zeta potential.</figcaption></figure>

<table><caption>**Table 1**
Experimental results from full factorial DOE.</caption><thead><tr><th colspan="4">Inputs</th><th colspan="3">Outputs</th></tr><tr><th>Run</th><th>Power</th><th>Speed (mm/s)</th><th>Height (mm)</th><th>Zeta</th><th>Abs Zeta</th><th>Conductivity</th></tr></thead><tbody><tr><td>1</td><td>1.3</td><td>10</td><td>5</td><td>-2</td><td>2</td><td>100</td></tr><tr><td>2</td><td>1.3</td><td>10</td><td>8</td><td>27</td><td>27</td><td>96</td></tr><tr><td>3</td><td>1.3</td><td>10</td><td>12</td><td>-33</td><td>33</td><td>105</td></tr><tr><td>4</td><td>1.3</td><td>100</td><td>5</td><td>-24</td><td>24</td><td>98</td></tr><tr><td>5</td><td>1.3</td><td>100</td><td>8</td><td>5</td><td>5</td><td>107</td></tr><tr><td>6</td><td>1.3</td><td>100</td><td>12</td><td>-2</td><td>2</td><td>105</td></tr><tr><td>7</td><td>1.3</td><td>1000</td><td>5</td><td>19</td><td>19</td><td>100</td></tr><tr><td>8</td><td>1.3</td><td>1000</td><td>8</td><td>-1</td><td>1</td><td>99</td></tr><tr><td>9</td><td>1.3</td><td>1000</td><td>12</td><td>30</td><td>30</td><td>97</td></tr><tr><td>10</td><td>1.3</td><td>3000</td><td>5</td><td>23</td><td>23</td><td>100</td></tr><tr><td>11</td><td>1.3</td><td>3000</td><td>8</td><td>15</td><td>15</td><td>63</td></tr><tr><td>12</td><td>1.3</td><td>3000</td><td>12</td><td>8</td><td>8</td><td>105</td></tr><tr><td>13</td><td>1.4</td><td>10</td><td>5</td><td>16</td><td>16</td><td>101</td></tr><tr><td>14</td><td>1.4</td><td>10</td><td>8</td><td>-14</td><td>14</td><td>101</td></tr><tr><td>15</td><td>1.4</td><td>10</td><td>12</td><td>-20</td><td>20</td><td>108</td></tr><tr><td>16</td><td>1.4</td><td>100</td><td>5</td><td>8</td><td>8</td><td>98</td></tr><tr><td>17</td><td>1.4</td><td>100</td><td>8</td><td>-12</td><td>12</td><td>94</td></tr><tr><td>18</td><td>1.4</td><td>100</td><td>12</td><td>2</td><td>2</td><td>102</td></tr><tr><td>19</td><td>1.4</td><td>1000</td><td>5</td><td>12</td><td>12</td><td>99</td></tr><tr><td>20</td><td>1.4</td><td>1000</td><td>8</td><td>2</td><td>2</td><td>99</td></tr><tr><td>21</td><td>1.4</td><td>1000</td><td>12</td><td>33</td><td>33</td><td>99</td></tr><tr><td>22</td><td>1.4</td><td>3000</td><td>5</td><td>-45</td><td>45</td><td>100</td></tr><tr><td>23</td><td>1.4</td><td>3000</td><td>8</td><td>10</td><td>10</td><td>106</td></tr><tr><td>24</td><td>1.4</td><td>3000</td><td>12</td><td>3</td><td>3</td><td>106</td></tr><tr><td>25</td><td>1.5</td><td>10</td><td>5</td><td>-4</td><td>4</td><td>101</td></tr><tr><td>26</td><td>1.5</td><td>10</td><td>8</td><td>19</td><td>19</td><td>101</td></tr><tr><td>27</td><td>1.5</td><td>10</td><td>12</td><td>17</td><td>17</td><td>109</td></tr><tr><td>28</td><td>1.5</td><td>100</td><td>5</td><td>-29</td><td>29</td><td>70</td></tr><tr><td>29</td><td>1.5</td><td>100</td><td>8</td><td>-36</td><td>36</td><td>100</td></tr><tr><td>30</td><td>1.5</td><td>100</td><td>12</td><td>30</td><td>30</td><td>97</td></tr><tr><td>31</td><td>1.5</td><td>1000</td><td>5</td><td>16</td><td>16</td><td>101</td></tr><tr><td>32</td><td>1.5</td><td>1000</td><td>8</td><td>-30</td><td>30</td><td>106</td></tr><tr><td>33</td><td>1.5</td><td>1000</td><td>12</td><td>-30</td><td>30</td><td>100</td></tr><tr><td>34</td><td>1.5</td><td>3000</td><td>5</td><td>-30</td><td>30</td><td>100</td></tr><tr><td>35</td><td>1.5</td><td>3000</td><td>8</td><td>-7</td><td>7</td><td>103</td></tr><tr><td>36</td><td>1.5</td><td>3000</td><td>12</td><td>-2</td><td>2</td><td>105</td></tr><tr><td>After Wet Chemistry</td><td></td><td></td><td></td><td>-18</td><td>18</td><td>99</td></tr></tbody></table>

Colloid conductivity exhibited slight variations post-PLAL compared to the wet chemistry batch, with the highest recorded conductivity reaching 108 mS/cm — close to the upper extent of lithium-ion battery electrolyte conductivity (0.1–100 mS/cm). Some instances recorded lower electrical conductivity values (63 and 70 mS/cm) post-PLAL than the wet chemistry colloids (99 mS/cm), emphasizing the imperative to fine-tune the PLAL process. Fig. 2d showcases the produced Cu colloids. Interestingly the height at which the laser is focused within the ablation media matters, a concept that has not been investigated extensively in the literature.

The investigation of particle connectivity was conducted using Field Emission Scanning Electron Microscopy (FESEM). Microstructure analyses of the electrodes, depicted in Fig. 5a–h, reveal the formation of channels through particles and rod-shaped glycerol strands, crucial for electrical conductivity. Notably, some gaps between channels or particles are observed in the microstructure (Fig. 5a–h). These gaps impede the flow of electrical current, consequently diminishing the conductivity of the electrodes. To address this issue, Cu colloids with the highest Zeta potential (45 mV) and a conductivity of 100 mS/cm were synthesized and applied as a coating onto copper electrodes at a concentration of 1 µl/cm². Remarkably, even with this small volume of Cu NPs, Fig. 5e and f illustrate nearly complete closure of the gaps. The high surface area and adhesive characteristics of NPs contribute to the effective closure of gaps, resulting in comprehensive coverage of Cu microparticles and glycerol-copper channels by NPs. A detailed examination of the surface reveals semi-spherical NPs adhering to microparticles (Fig. 5g), while a high-resolution image in Fig. 5h showcases a cluster of NPs enveloping the apex of a copper microparticle. This arrangement forms a homogeneous nanostructure characterized by nearly seamless connectivity among nanoparticles.

The EDX analysis results for the electrodes, both pre- and post-nanoparticle coating, are presented in Figs. 6 and 7, respectively. Fig. 6 illustrates the presence of Cu, C, O, and Cl within the electrodes. The blob-like structures primarily consist of Cu, indicating the tendency of copper microparticles to aggregate and form clusters, thereby contributing to electrical conductivity. The rod-like channels are predominantly composed of C and O, associated with the presence of glycerol molecules. Fig. 7 reveals a more interconnected and homogeneous microstructure within the electrodes following the incorporation of Cu NPs. This enhanced structure is notably more complex and
<figure><img src="image_8.png" /><figcaption>Fig. 5. FESEM images of copper electrodes a-d) before copper nanoparticle coating and; e-h) after copper nanoparticle coating.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. EDX analysis of copper electrodes without copper nanoparticle coating.</figcaption></figure>

interconnected compared to the configuration without NPs (Fig. 6). The EDX analysis now indicates a significant increase in Cu content within the rod-like structures. Furthermore, the chlorine distribution pattern is now following a similar geometry to that of C, attributed to the incorporation of Cl-containing Cu NPs directly onto the electrode surfaces.

Fig. 8 illustrates the electrical resistivity of electrodes both with and without the presence of Cu NPs. The introduction of Cu NPs onto the electrode surfaces resulted in a remarkable reduction in resistivity, exceeding a fourfold decrease. This improvement can be ascribed to the heightened electrical conductivity exhibited by Cu NPs, coupled with increased surface area, an augmented number of charge carriers, and enhanced connectivity, as corroborated by FESEM analysis.

In another experiment, Cu NPs were deposited onto plain paper substrates to investigate their conductivity. The deposition of Cu NPs onto paper substrates was achieved through a straightforward dip-coating technique, as depicted in Fig. 8c. A range of Cu NP concentrations was experimented with, and the impact of concentration on resistivity was examined using 4-Point probe analysis. In Fig. 8c, the gradual decrease in NP concentration is evident through the fading of the blue color. As anticipated, the concentration of NPs influences resistivity. It is noteworthy that concentration is not the sole contributing

factor; it is essential to consider the various concentrations prepared using a water dilution technique. The water molecules themselves may induce oxidation or chemical instability, thereby reducing the conductivity of the original NPs.

## 5. Discussion

This study showcases screen-printing as an effective method for swiftly producing flexible and cost-efficient electrodes applicable to battery electrodes, sensor electrodes, and electrical circuits owing to the conductivity and connectivity of the particles. A critical aspect identified is the viscosity of the printed inks, serving as the primary optimization parameter. Screen-printing demands a precise viscosity range, challenging to achieve with microparticle or nanoparticle inks in basic liquids like water or organic solvents. Glycerol emerged as a practical solution to finely adjust ink viscosity, transforming it into a nearly paste-like consistency.

Following viscosity optimization, the next crucial parameter is particle connectivity. The remarkable enhancement in particle connectivity after screen-printing was achieved through the incorporation of Cu NPs. This additional step resulted in an over four-fold increase in electrical
<figure><img src="image_10.png" /><figcaption>Fig. 7. EDX analysis of copper electrodes with copper nanoparticle coating.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Resistivity of copper electrodes composed of microparticles and microparticles + nanoparticles; b) effect of copper nanoparticle concentration and water dilution on the resistivity of paper substrates, and c) dip-coating method for paper substrates before resistivity tests.</figcaption></figure>

conductivity, establishing its necessity in the screen-printing process. Nanoparticles, approximately 50 nm in size, were observed on the surfaces of copper microparticles and glycerol channels due to the high surface area-to-volume ratio. The binding of nanoparticles to electrodes is known to be facilitated by multiple factors:

1. Accommodating paper substrates absorbing particles.

2. Van der Waals forces between NPs, microparticles, and glycerol molecules.

3. Electrostatic forces between negatively charged nanoparticles and positively charged copper microparticles.

4. Complementary surface energies of materials.

5. Surface roughness of the electrode surface creates cavities for nanoparticle residence and movement.

6. Chemical interactions and glycerol's adhesion properties contribute to NP attachment.

The synergy of these factors, combined with the inherent properties of copper and glycerol, results in high conductivity and particle connectivity, making these electrodes competitive and versatile. Moreover, the use of readily available and cost-effective materials like paper, copper, and glycerol positions them as sustainable alternatives to the

commonly used silver, lithium and plastic electrodes in printed electronics.

It has been reported that metallic nanoparticles have an affinity to polymer structures, forming a polymer bridge that connects the micro-particles to the NPs. Bailey et al. demonstrated the microparticle-polymer-nanoparticle composite using various metallic NPs including TiO₂, Fe₂O₃ and SrTiO₃, onto SiO₂ microparticles [19]. Various polymers were tested including poly (pentafluorophenyl acrylate) and in all cases, an electrostatic component is introduced via the presence of primary amine groups, which remain positively charged over various pH conditions enabling the polymer to attract negatively charged particles such as nanoparticles. The same research team functionalized NPs with polymers to enable strong attachment with microswimmers owing to the availability of functional groups [20]. Glycerol contains three hydroxyl (-OH) functional groups attached to each C atom, making it a strong attractor of the metallic Cu NPs and has been used for metallic NP immobilization for catalytic applications, reaching a high immobilization yield above 90% [21]. Glycerol itself is a triol monomer, making it generally more reactive than polymers and is expected to have higher particle attachment abilities than polymers. The NPs herein were found to be highly positively charged with high Zeta potentials which promotes reactivity/attachment with glycerol's -OH functional groups.
Additionally, it has been reported that the Cu NPs can react to glycerol molecules upon attachment leading to the reduction of the Cu particles to Cu₂O [22]. According to the FTIR analysis, the electrodes exhibited various functional groups which are available for NP attachment. Moreover, due to its monomeric nature, glycerol does not inherently develop fiber-like microstructures. Nonetheless, the presence of fiber channels was identified through FESEM, and this phenomenon is associated with the presence of micro and nanoparticles, which have a tendency to agglomerate or adhere to each other.

The integration of wet chemistry with PLAL in Cu NP synthesis was intriguing, marking this article as the first to document such a method. While the literature recognizes the ability of the PLAL process to modify NP size, its impact on conductivity and stability (Zeta potential) has been sparingly explored, rendering this study unique. A diverse range of Zeta potentials (both positive and negative) was attained through PLAL by adjusting the laser's focal height within the ablation medium and the scanning speed. Notably, higher focal heights, measured from the bottom of the ablation vessel, were found to enhance NP stability, particularly at scan speeds ranging from 100 to 2000 mm/s. The laser power, ranging between 1.3 and 1.5 W, proved negligible, allowing for energy conservation by utilizing the lowest power setting (1.3 W), a crucial consideration in continuous or long-term manufacturing operations. Although the powder difference between 1.3 and 1.5 W seems modest (around 23%), its significant impact in large-scale settings underscores the importance of judicious energy use.

This study underscores the imperative of process optimization to conserve energy, time, and materials. The establishment of the minimum Cu NP concentration required for achieving specific particle connectivity and conductivity remains a key objective in optimizing material utilization. While further research is warranted, this study has already introduced a novel material combination capable of rapidly producing electronics, including printable batteries. Moreover, the versatility of this technique allows for multi-material printing, paving the way for advanced materials. Additionally, although this study involved the printing of a single layer for each electrode, the ease of re-printing on the same spots facilitates multiple layers (3D printing), offering the potential for multi-material parts and enhanced connectivity. The optimization of both the NP fabrication process and the screen-printing process can easily generate terabytes of data (including images of electrodes after printing, videos of the printing process, and numerical data such as sheet resistivity) which begs the need for machine learning algorithms to attain valuable data.

## 6. Conclusion

In summary, this study marks a pioneering effort in detailing the development of copper microparticle-nanoparticle-based electrodes on paper substrates. Leveraging copper's advantageous characteristics, such as high electrical and thermal conductivity, positions it as a promising material for the fabrication of printable and flexible low-cost batteries and sensors. Through the incorporation of a small volume of Cu nanoparticles (1 µl/cm²), the sheet resistance of the microparticle electrodes experienced a significant 4-fold reduction, dropping from approximately 200 to around 50 Ω/m². Additionally, the investigation revealed a noteworthy correlation between the nanoparticle loading and the decrease in paper's sheet resistivity, underscoring the potential for further enhancements in electrode conductivity. The optimization process involved fine-tuning the ink viscosity, achieving an optimal value of 62 mPa s at a microparticle to glycerol ratio of 1.95:1. It is crucial to prioritize viscosity as the initial parameter, followed by particle connectivity and electrode conductivity. Notably, post-processing techniques such as sintering and the addition of nanoparticles, as demonstrated in this study, offer avenues for further optimizing electrode conductivity. This research opens the door to continued advancements in the development of high-performance, cost-effective, and flexible electronic devices.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgment

This publication has emanated from research [conducted with the financial support of/supported in part by a grant from] Science Foundation Ireland under Grant number [16/1571 RC/3872 and 19/US-C2C/3579]. For the purpose of Open Access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission. This work is supported by I-Form.

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