2.1. Process

The sample preparation process is schematically illustrated in Fig. 1. Prior to electrodeposition, SUS 304 (POSCO, 13 cm × 13 cm) was treated with an alkaline cleaner (BGF, IRIS) at 50 ℃ for 30 min to remove organic substances. The substrate was then cleaned with deionized water and immersed in 10% H₂SO₄ (Samjeon Pure Chemical Industries) at 25 ℃ for 3 min to perform acid rinsing. H₂SO₄ was used instead of HCl to avoid Cl- contamination. The bath initially consisted of 0.5 M Cu(MSA)₂ (MSC Corp.) 0.5 mM Ag(MSA) (MSC Corp.) and 1.0 M MSA (Sigma Aldrich). The solid-state precipitate (possibly AgCl) in the solution was removed using a filtration system (Jungdo Corp.). For activation of the electrolytic solution, electrodeposition was carried out in a 3 L bath by applying a constant current (either 25, 50, 75, or 100 mA/cm²) using the SUS 304 substrate as the cathode and an IrO₂-coated Ti plate as the anode (exposed area: 270 cm²). During activation, Ag⁺ was constantly added into the bath at 0.015 mmol/min using a liquid pump. The temperature of the bath was maintained at 25 ℃ using a jacket-bath system. The dummy foil prepared via the activation process is shown in Fig. 1.

Fig. 1. Experimental procedure in this study.

After activation, CuAg foils were prepared via the pulsed current electrodeposition technique. The cleaned SUS 304 was loaded on a homemade jig (exposed area: 121 cm²) and used as the cathode. An IrO₂-coated Ti plate with an exposed area of 270 cm² was used as the counter electrode. A peak current density of 50 mA/cm² was periodically applied with a frequency of 0.5 Hz and a duty cycle of 50%. The total amount of deposition was fixed at 72 C/cm². During electrodeposition, the solution was stirred constantly and the temperature of the bath was maintained at 25 ℃ using a controller. Ag⁺ was constantly added into the bath at a rate of 0.015 mmol/min using a liquid pump. After the electroplating process, the deposited foil was immersed in a chromate solution (0.5 M CrO₃) for 2 min to avoid oxidation. The deposited foil was then separated from the substrate, washed with distilled water, and stored in a vacuum chamber before analysis.

2.2. Analysis

Cyclic voltammetry was carried out in a three-electrode system consisting of a Pt rotating disk electrode (active area: 0.197 cm²), Pt rod, and Ag/AgCl (3M KCl) reference electrode. Prior to analysis, electrode conditioning was performed five times through potential cycling between 1.5 V and 0 V with a scan rate of 100 mV/s. After conditioning, cyclic voltammetry was performed with a scan rate of 10 mV/s between 1.5 V and 0 V or between 1.0 V and -0.3 V. The solution temperature was maintained at 25 ℃ using a thermostat.

To evaluate the foil thickness, the mass of the CuAg foil was measured using an electronic balance (METTLER TOLEDO), and the mean thickness was evaluated through the weighing method. The details of the thickness evaluation method can be found in our previous paper^[15].

For the tensile test, the CuAg foil was cut into a rectangular shape (120 mm × 12.7 mm × 0.016 mm) using a custom-made cutter. The foil was then loaded on a tensile machine (EZ-L 5 kN, Shimadzu) with a gauge distance of 30 mm. The test was carried out four times with a strain rate of 2 mm/min at room temperature (~25 ℃). The conductivity of the foil was measured using a four-point probe (MCP-T600, Mitsubishi) at room temperature.

The morphologies and oxidation states of CuAg were examined using a field emission scanning electron microscope (FESEM, SU-6600) coupled with an energy dispersive X-ray spectrometer (EDS) and an X-ray photoelectron spectroscope (XPS, Multilab-2000). The microstructure of the CuAg foil was analyzed via X-ray diffraction (XRD, D/Max-2500VL (Rigaku International Corp.)) using copper as the X-ray source at a 2θ of 20–80° with a scan rate of 3°/min under room temperature. All analyses were performed one week after the electrodeposition.

3.1. Effect of electrolytic solution activation

Figure 2 shows the morphologies of CuAg foils prepared in non-activated and activated baths. As shown in Fig. 2(a), the CuAg foil from the non-activated bath has a matted and irregular surface with a high density of micro-scale nodules. As shown in Figs. 2(b)-(e), solution activation smoothens the deposit surface and reduces the nodule frequency. A nodule-free, mirror-bright surface was obtained using the activated solution for 120 min (Fig. 1(e)) at 50 mA/cm². As shown in Figs. 2(f) and (g), both the strength and conductivity of the CuAg foil gradually improve through electrolytic solution activation. This simultaneous improvement indicates that the density of the stress raiser (e.g. voids) decreases with solution activation. Electrolytic activation was performed with various current densities (25–100 mA/cm²), and the properties of the CuAg foil obtained from the activated bath are depicted in Fig. 3.

Fig. 2. (a–e) Photograph and FESEM images of CuAg foils electrodeposited from (a) a non-activated bath and (b–e) an activated bath with a current density of 50 mA/cm² and activation times of (b) 30 min, (c) 60 min, (d) 90 min, and (e) 120 min. (f) Stress–strain curves and (g) ultimate tensile strength (UTS) and electrical conductivity of the CuAg foils shown in (a–e).

Fig. 3. (a) UTS and (b) electrical conductivity of CuAg foils obtained from non-activated and activated baths (current density for activation: 25, 50, 75, and 100 mA/cm²).

Although there were some deviations depending on the current density, both the strength and conductivity simultaneously improved with an increase in the consumed charge. This also demonstrates that the solution activation effectively suppresses irregular growth of the CuAg foil.

The bath composition changes after activation were examined to investigate the cause of the improved properties. An electrochemical analysis was performed to confirm the change in the concentrations of Ag⁺ (C_Ag+) and Cu²⁺ (C_Cu2+) after the activation process (Fig. 4).

The cyclic voltammetry (CV) results obtained from the non-activated CuAg bath with various Ag⁺ concentrations show two characteristic redox processes (Ag⁺/Ag⁰ and Cu²⁺/Cu⁰) in a potential range of 0–0.6V (vs. Ag/AgCl). As shown in the inset of Fig. 4(a), the limiting currents associated with the reduction of Ag⁺ to Ag⁰ appear between 0.15 V and 0.25 V. The limiting current density (obtained at 0.2 V) is proportional to the Ag concentration (C_Ag+) (Fig. 4(b)). The C_Ag+ of the activated bath was determined using Fig. 4(b) as a calibration plot (Fig. 4(d)). Here, the concentration of Cu²⁺ was obtained using the Eqs. (1) to (3), assuming a cathodic current efficiency of 100%.

where Q_total is the total consumed charge (C), Q_Cu2+ is the partial charge consumed by Cu²⁺ reduction (C), Q_Ag+ is the partial charge consumed by Ag⁺ reduction (C), $m^c_{Ag+}$ is the amount of consumed Ag⁺ ion (mol), F is the Faraday constant (C/mol), $m^t_{Ag+}$ is the amount of Ag⁺ ions in the bath (mol), $m^0_{Ag+}$ is the initial amount of Ag⁺ in the bath (mol), $r_{Ag+}$ is the supplement rate for Ag⁺ ions (mol/s), and t is the time (s). The Ag⁺ concentration decreased from 0.5 mM to 0.17 mM after activation at 50 mA/cm² for 120 min. Simultaneously, the Cu²⁺ concentration decreased from 0.5 M to 0.41 M.

Fig. 4. (a) Voltammograms for baths containing 0.5 M Cu(MSA)₂, x (x = 0, 0.125, 0.250, 0.375, 0.500) mM Ag(MSA), and 1.0 M MSA. The inset shows a magnified image of (a) between 0.1 and 0.5 V. (b) Limiting current density for Ag⁺ reduction (with current density at 0.2 V) depending on Ag⁺ concentration. (c) Changes in Ag⁺ and Cu²⁺ concentration during activation.

To clarify the effect of the electrolyte concentration on the growth of CuAg, CuAg foils were obtained from solutions of 0.41 M Cu(MSA)₂ + 0.17 mM AgMSA + 1.0 M MSA without activation. The CuAg foil from low Ag⁺ concentration shows a lower nodule density than that from high Ag⁺ concentration (Fig. 2(a) and Fig. 5).

Fig. 5. (a, b) (a) Photograph and (b) FESEM images of CuAg foil obtained from 0.41 M Cu(MSA)₂, 0.17 mM Ag(MSA), and 1.0 M MSA. (c, d) Magnified images for (c) nodule and (d) flat surface, as well as the local composition.

This demonstrates the importance of the Ag⁺ concentration in the electrolyte for CuAg growth. However, compared to the CuAg foil from the activated bath (Fig. 2(e)), the surface is dull with some nodules present, indicating the possibility of other important factors affecting CuAg growth. As shown in Figs. 5(c) and (d), the Ag content of the nodule is 6.64 wt%, which is higher than that of the flat surface (1.34 wt%).

3.2 Analysis of dummy sample

One possible effect of solution activation is a decrease in the impurity level in the solution. To clarify the impurity content, a dummy sample (CuAg foil made via the solution activation process, see Fig. 1) was analyzed via FESEM-EDS. The surface of the dummy sample presented an uneven and irregular morphology with a high density of nodules at the center.

Figures 6(a)-(c) show the FESEM-EDS results for the surface underneath the nodule, the nodule, and the growth front of the nodule (details in Fig. 6(d)). The EDS results for the region underneath the nodule (Fig. 6(a)) and the nodule (Fig. 6(b)) show Cl contents of 0.51 wt% and 0.22 wt%, respectively, indicative of Cl^- impurity in the solution. The EDS results also show that the Ag-rich phase nucleates at the first stage of nodule growth (Fig. 6(a)). The core of the nodule constitutes round-shaped Cu-rich phases, upon which the dendrite-shaped Ag-rich phase grows (Figs. 6(b) and (c)). The disordered growth pattern for the dendrite stem and branch indicates mass-transport-limited dendrite growth^[21].

Fig. 6. (a-c) FESEM-EDS results for (a) the surface beneath the nodule, (c) nodule, and (c) the magnified image of the nodule. (d) Schematics of the structure of nodule based on FESEM-EDS results.

The XRD results for the dummy sample underneath the nodule show peaks corresponding to Cu(111), Cu(200), and Cu(220) (Fig. 7).

Fig. 7. XRD results for the surface beneath the nodule and nodule powder.

All the peaks are slightly shifted in the negative direction compared to pure Cu, indicative of a CuAg solid-solution state. The formation of a non-equilibrium solid-solution has been typically observed for electrodeposited CuAg ^[11,13]. The peaks for the Ag-rich phase cannot be observed, possibly owing to low densities. Meanwhile, the XRD results for the nodule additionally show an Ag(111) peak at 38.2°, indicative of the formation of a crystalline Ag-rich phase on the nodule. This is consistent with Fig. 6(c), where an Ag-rich phase with a disordered dendrite shape can be observed.

The chloride content in the dummy foil demonstrated that Cl^- ions dissolved in the pristine CuAg bath. The Cl^- impurity can be attributed to MSA because Cl^- was used as a reactant for the synthesis^[25]. We attempted to analyze the concentration of Cl^- ions in the bath using ion selective electrode and ion chromatography methods. Both methods, however, failed to detect Cl^-, possibly due to a low Cl^- concentration and concentrated matrix. Considering the solubility constant of AgCl (1.6*10^-10 at 25 ℃), it is likely that Cl^- ions with 3.2*10^-7 M dissolved in the solution. Meanwhile, the Cl^- can be eliminated during electrochemical activation through either cathodic incorporation or an anodic oxidation mechanism. The observation of Cl impurities in the dummy sample supports the Cl^- incorporation mechanism (Fig. 6). In addition, previous studies have reported that the chlorine evolution reaction can occur under similar conditions, indicating that an anodic oxidation mechanism may also contribute^[26,27]. The removal of Cl^- ions was confirmed through the XPS results of CuAg foils obtained from non-activated and activated solutions, where the 2p was reduced after the solution activation process (Fig. 8).

Fig. 8. XPS results for CuAg foil obtained from non-activated and activated bath (current density for activation: 50 mA/cm², activation time: 120 min).

3.3. Mechanism of irregular growth

One characteristic of nodule formation in our study is the Ag-rich phase at the beginning of nodule growth (Fig. 6(a)). The FESEM-EDS images for the region beneath the dendrite powder show an inhomogeneous distribution of Ag atoms, wherein some Ag clusters are observed. This indicates an Ag-rich phase is formed at the beginning of dendrite growth. The formation of the Ag-rich phase is accelerated at the growth front, as seen in the FESEM and XRD results. Another characteristic feature is the presence of chloride impurities in the dendrite powder. Chloride impurities were not detected on the compact CuAg surface, indicating that the chloride contributed significantly to dendrite formation.

In general, the metal adatoms formed by charge transfer can be crystallized by nucleation or incorporation in the nearby crystal lattice (crystal growth). From this perspective, there are four possible avenues for the crystallization of Ag⁰: nucleation as Cu-rich or Ag-rich phases or the incorporation of nearby Cu-rich or Ag-rich crystals. The uniform Ag distribution observed in compact CuAg as well as the XRD results shown in Fig. 7 indicate that Ag⁰ is mainly crystallized by incorporating the Cu-rich phase. Meanwhile, the non-uniform Ag distribution in the dendrite structure demonstrates nucleation and growth of the Ag-rich phase (Fig. 6(a)).

The formation of the Ag-rich phase on the surface can result in a dual-phase anisotropic surface consisting of Cu-rich and Ag-rich phases. The anisotropic surface leads to variation of the local reduction rate due to the surface catalytic effect. This was supported by the voltammetry results obtained at -0.3–0.4 V (Fig. 9).

Fig. 9. LSV results for (a) 0.41 M Cu(MSA)₂ + 1.0 M MSA and (b) 0.41 M Cu(MSA)₂ + 0.2 mM Ag(MSA) + 1.0 M MSA with various rotating speeds. (c) CV results with various Ag⁺ concentrations at a rotating speed of 2000 rpm.

As presented in Fig. 9(a), the LSV curves for the Ag⁺-free bath show an activation control region at low overpotential (region i) and a mixed control region at high overpotential (region ii). In the presence of 0.2 mM Ag⁺, diffusion-controlled Ag⁺ reduction is observed between 0.1 V–0.3 V (inset of Fig. 9(b)). This is followed by an abrupt increase in the cathodic current density in region i that demonstrates the catalytic effect of Ag⁰ on Cu²⁺ reduction. When the potential shifts from -0.05 to -0.15 V, the cathodic current decreases because the electrode surface is converted from the Ag-rich phase to the Cu-rich phase. The height of the peak at around -0.06 V is almost independent of the rotating speed, which indicates the catalytic effect is dominated by the surface state. This result was consistent with previous studies that reported voltammetry results for a CuAg bath in sulfate and methanesulfonate media^[15,28].

In the high overpotential region (-0.15 V to -0.3 V), the presence of Ag⁺ results in a low cathodic current (Fig. 9(b)). In the case of the reverse scan, when the electrode surface is fully covered by the Cu-rich phase, the suppression of Cu²⁺ reduction by Ag⁺ became evident (Fig. 9(c)). This shows a significant anisotropy in Cu²⁺ reduction kinetics on Cu-rich and Ag-rich phases. The data are consistent with our results where the formation of the nodule is driven by the Ag-rich phase (Fig. 6).

On the nodule surface, the acceleration of the nucleation of another Ag-rich phase is attributed to high Ag⁺ mass flux. This can result in the formation of another nodule at the primary nodule surface. The diffusion-limited growth of Ag is supported by the disordered Ag-rich dendrite structure observed at the growth front (Fig. 6(c)). It is also supported by the convection-dependent Ag⁺ reduction in the CV results (Fig. 9(b)).

Based on our assumption, the high Ag⁺ concentration may be the primary factor underlying the irregular growth of CuAg, because it results in a high probability that the Ag-rich phase will form. However, surface images of the CuAg foil prepared with a low C_Ag+ bath (Fig. 4) also show this irregular growth. This demonstrates that another factor also affects the phase separation during the growth of CuAg. As seen in Fig. 6, chloride is detected in the dummy sample, implying that the non-activated bath contains chloride impurities. Considering Fig. 8, the chloride impurities were removed during the activation process. Therefore, the chloride ions could accelerate the formation of the Ag-rich phase (or phase separation) during CuAg growth.

The effect of chloride ions on irregular film growth has been examined in various studies. Berkely et al explained that kinetic anisotropy in Cu electrodeposition could be attributed to chloride ions because they catalyzed Cu²⁺ reduction depending on the surface orientation^[23]. Shao et al. reported that the presence of excess chloride in the electrodeposition bath resulted in dendrite growth during electrodeposition^[21]. This is because Cu²⁺ reduction rapidly occurs at the {110} planes where the surface concentration of chloride is the highest. However, our non-activated Ag-free bath yielded compact features with reasonable tensile properties, and thus the irregular growth should not be ascribed to the anisotropic adsorption of chloride on Cu surface (Fig. 1(f)).

Considering previous studies, nucleation of the Ag-rich phase could be accelerated in the presence of chloride ions. As Cu and Ag are immiscible at room temperature, the phase separation is thermodynamically favorable. Hence, when the mean-free path of Ag⁰ adatoms on the surface is sufficiently high, the formation of the Ag-rich phase can be accelerated^[19]. Here, the diffusion rate of adatoms on the surface is a function of the surface dipole and thus depends on the electrode potential and other adsorbed species such as halides^[29-31]. In particular, in the presence of halides, enhanced surface diffusion of metal atoms has been reported^[29,[32,33]. Considering previous studies, the diffusion of Ag on a Cu-rich phase could be accelerated on a chloride-covered Cu-rich phase, followed by the nucleation of an Ag-rich phase. It is likely that the nucleated Ag-rich phase provides catalytic sites for dendritic CuAg growth.

The proposed mechanism of dendrite growth is shown in Fig. 10.

Fig. 10. Suggested mechanism for the irregular growth of CuAg.

For a compact CuAg foil, Ag⁰ adatoms should be crystallized by incorporating a Cu-rich phase. However, at high Ag⁺ concentrations or in the presence of chloride impurities, the nucleation of the Ag-rich phase (or the segregation of Ag) at the surface was accelerated. The inhomogeneous Ag composition resulted in irregular growth of deposits and nodule formation. At the nodule, the diffusion flux of Ag⁺ increased, thereby providing positive feedback for Ag nucleation. When the bump grew to a few tens of μm, the diffusion flux of Ag⁺ was concentrated at the growth front, whereby the dendrite of the Ag-rich phase could be formed.

Our study shows that tightly controlling C_Ag+ as well as chloride removal is necessary to suppress dendrite growth in the electrodeposition of CuAg. In addition, electrolytic solution activation is effective for chloride removal. Our findings point toward applications in fields where a compact CuAg structure without defects is desirable, such as in the automobile and electronic industries.