Properties and microstructure regulation of electrodeposited ultra-thin copper foil in a simple additive system

Hydroxyethyl cellulose (HEC) has been commonly used in a variety of complex formulations for acid copper plating. However, the roles of HEC acting in acid copper plating still lacks of systematic investigation. To explore the efficacy of HEC in the deposition of the ultra-thin electrodeposited copper foil (ED-Cu), we designed a simple formulation system, in which HEC was used as the single organic additive. Using electron backscatter diffraction (EBSD), microstructures of the prepared ED-Cu was comprehensively investigated. The results showed that the ED-Cu was characterized by a mixed distribution of columnar and equiaxed crystals. Grain morphology, dislocation density and crystal orientation of the ED-Cu could be regulated by HEC concentration. According to the cyclic voltammetry (CV) and chronoamperometry (CA) results, the introduction of HEC between 0–200 ppm led to a polarizing effect, which marginally increased with the HEC concentration. Meanwhile, the increase of HEC concentration enhanced the nucleation rates of copper and reduced the grain size during instantaneous nucleation. The introduction of the HEC also altered the preferred orientation of the ED-Cu foil. Mechanical results showed that the optimum concentration of HEC addition was 125 mg l−1.


Introduction
Nowadays, lithium-ion batteries play a vital role in the development of the 3 C industry and the emerging electric vehicle sector.In this case, how to enhance energy density and extend cycle life of the batteries becomes a great challenge.To meet with this challenge, research focusing on lightweight construction of Lithium-ion batteries should be taken [1][2][3][4].As a key ingredient of cathode in the lithium-ion batteries (LIBs), electrodeposited copper foil (ED-Cu) affects the electrochemical performance and the manufacture of the batteries to a great extent.The development of ED-Cu in applications tends to be thinner and lighter, which helps in increasing the energy density of LIBs [4][5][6].
On the ED-Cu production line, copper sulphate and sulfuric acid system is the most widely used copper plating system for decreasing the whole expenditures.During the ED-Cu production process, the most crucial step is the reduction reaction on the cathode roll under direct current, which determines the surface properties of the resulting ED-Cu.There are lots of factors that will influence the quality of the final ED-Cu, such as deposition parameters [7], composition of the electroplating solution [8][9][10][11], additives [12], etc All these factors will cause differences of surface quality of ED-Cu.Among these factors, additives take up an extremely important position.Additives are usually classified as wetting agents, leveller, brightener, and chloride ions (Cl − ) according to their effects.Brighteners are mainly sulfur-containing organics [13,14], levellers are mainly quaternary ammonium-containing polymer [15][16][17][18] or small molecule dyes [19], and wetting agents are mainly polyether compounds [20,21].Wetting agents contribute to the enhancement of plating solution wettability, reducing interfacial tension.These features have profound effects on the improvement of surface defects, notably pinholes and pockmarks, therefore lead to the overall quality of ED-Cu [22,23].Hydroxyethyl cellulose (HEC), as a wetting agent, has been widely used in ED-Cu production formulations [24,25] due to its excellent thermal stability, dispersibility, solubility and cost advantages [26].The function of HEC in electrolytes has been the subject of several studies.El-Haddad [27] and Mobin [28] found that the adsorption of HEC on carbon steel substrates followed the Langmuir adsorption isotherm.Zhao [29] showed electrochemically that HEC has a polarizing effect in the electrolyte of the acid-copper system, impeding the reduction process of copper ions.
Despite the wide use of HEC in production of ED-Cu, the formulation of HEC is predominantly experienced-driven, lacking systematic investigation.Few studies have been conducted to investigate the effect of different concentrations of HEC on ED-Cu especially the evolution of microstructure in ultra-thin ED-Cu.Therefore, this study investigates the mechanism of HEC in copper deposition from the electrochemical mechanism and microstructure scale to provide theoretical guidance for optimizing the production of ED-Cu.

Experiments
2.1.Ultra-thin ED-Cu preparation and properties ED-Cu with a thickness of 6 μm was prepared by DC electrodeposition.The conditions of virgin makeup solution (VMS) were 90 g l −1 Cu 2+ , 120 g l −1 H 2 SO 4 , and 30 mg l −1 Cl − .HEC was dissolved in ultrapure water to achieve a concentrated and homogeneous solution with a concentration of 10 g l −1 .Subsequently, it was added to the VMS as needed to formulate an electrolyte with HEC as the only organic additive (VMS + 0-200 mg l −1 HEC).Polished TA1 titanium plates were used as cathodes and ruthenium-iridium titanium plates were used as anodes.The exposed area of titanium plate cathode was 90 mm × 40 mm, while the remaining positions and edges were taped.The distance between cathode and anode was 10 mm.The current density during electrodeposition was kept constant at 60 A/dm 2 .The electrolyte temperature was maintained at 54 ± 0.5 °C, the flow rate was set at 5 l min −1 , and the electrolyte flow direction was from the bottom to the top of the cathode plate to ensure sufficient diffusion of copper ions and additives.After electrodeposition, the ED-Cu was removed from the cathode with adhesive tape and then rinsed with deionized water to wash off any residual plating solution.ED-Cu was then blown dry with cold air, and then placed in a sample bag and stored in a constant temperature drying oven.Here, the prepared ED-Cu has one side with a shiny surface (S side) where it was removed from the titanium substrate, and the other side is a matte side (M side).Place the ED-Cu under natural light and use a digital camera to photograph the external appearance of the ED-Cu.Place the ED-Cu flat on the surface of a strong white light source and record the light transmittance conditions of the ED-Cu.The surface roughness of ED-Cu was measured using a roughness tester (PS10, Mahr), the probe was tested for roughness on the M-side of ED-Cu over a lateral reciprocation of 4.8 mm, and three tests were performed for each sample, the average value was taken and the error value was calculated.

Microstructural characterization
Before microstructural characterization, the ED-Cu was immersed in 10% sulfuric acid for 30 s to remove the oxide layer on the surface of the ED-Cu.After immersion, the ED-Cu was washed with deionized water, then transferred to anhydrous ethanol for cleaning, which was repeated three times for 10 min to remove the dirt from the surface of ED-Cu.The micromorphology of the ED-Cu (M side) was characterized using a scanning electron microscope (SEM, ZEISS GeminiSEM460), and the microstructure of the cross-section of the ED-Cu along the deposition direction was characterized using electron backscatter diffraction (EBSD) at an accelerating voltage of 20 kV with a step size of 50 nm.EBSD samples were polished with argon ions (Gatan 685 PECS II) to obtain a stress-free viewing surface.EBSD data were analysed using AztecCrystal software.

Electrochemical methods
The electrochemical tests were performed in an electrochemical workstation (Reference 620, GAMRY) using a three-electrode system with a 1 cm 2 × 1 mm platinum sheet electrode as the working electrode, a 2 cm 2 × 1 mm platinum sheet electrode as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode.Cyclic voltammetry (CV) was performed by negatively sweeping from 1.5 V (versus SCE) to −0.25 V (versus SCE) and back to the initial potential for one cycle at a scan rate of 100 mV s −1 .CV test electrolyte conditions were 45 g l −1 Cu 2+ , 60 g l −1 H 2 SO 4 , and 30 mg l −1 Cl − at a temperature of 25 ± 0.5 °C.Chronoamperometry (CA) was performed at VMS and 54 ± 0.5 °C with overpotential of −0.1 V (versus SCE) and a deposition time of 50 s.

Result
Figure 1 shows the appearance of the matte and shiny surfaces of the ED-Cu, and the photographs were taken respectively, as shown in figures 1(a), (b).The ED-Cu exhibited a light pink hue overall.However, within the range of 0-200 mg l −1 HEC addition, various appearance defects are observed, including scorching at the top, pinholes, translucent areas, and coloured patches.The scorching at the top was caused by the shedding of copper powder during the cleaning process after plating, and the generation of copper powder is due to the hydrogen reaction [30].Pinholes are small holes with diameters of 30-400 μm that pass through the ED-Cu, which was caused by non-conductive impurities adsorbed on the cathode or by hydrogen bubbles that do not desorb immediately [23].Translucent areas were formed by multiple pinholes for reasons similar to pinhole formation.Colour patches were an area that differs in colour from the surrounding plating and can be attributed to abnormal copper deposition.In the HEC-free condition, the ED-Cu showed pinholes and translucent areas.Interestingly, the pinhole defects showed a significant reduction trend with the gradual addition of HEC, indicating that the addition of HEC on the HEC-free suppressed the pinhole defects.At a HEC concentration of 30-40 mg l −1 , the appearance showed a colour patch.Further addition of HEC concentration to 50 mg l −1 , it was suitable concentration to obtain ED-Cu with basically no defects on the appearance.In addition, it should be noted that the HEC concentration of 200 mg l −1 resulted in further expansion of the deficiencies area of the ED-Cu, which is considered an excessive concentration.
Three-dimensional profiles and the micromorphology of ED-Cu matte surface are shown in figure 2. Figure 2(a) shows the decrease of roughness with the increase of the HEC concentration, which is most pronounced below 40 mg l −1 and then remains in a relatively flat interval.When the HEC concentration exceeded 100 mg l −1 , the roughness began to increase slowly.A sharp decrease in roughness with the addition of 0-40 mg l −1 HEC to the VMS is associated with the elimination of translucent areas and the reduction of pinhole defects.e2), with the addition of HEC, the hemispherical copper particles protruding from the surface of ED-Cu become progressively more obvious.Overall, as the HEC concentration increased, the lustreless surface showed a transition pattern from wavy with the condition of HEC-less, to a smoother state with the condition of HEC-suitable, and finally back to wavy with the condition of HEC-excessive.
Based on the observation of ED-Cu, four different HEC concentrations were added to VMS, and the effect of HEC on the microstructure of the ED-Cu was studied.EBSD analysis was used to get information such as grain boundary distribution characteristics and orientation.Figure 3 shows the grain boundary distribution of the  (c)), coarse columnar crystals were observed to become progressively thinner and equiaxed crystals were observed to become progressively more numerous, smaller and more agglomerated.This effect was more pronounced at a concentration of 200 mg l −1 HEC (figure 3(d)).Overall, the grain boundary distribution in the cross-section of ED-Cu along the growth direction was characterized by the mixed growth of columnar crystals and equiaxed crystals, with fine equiaxed grains diffusely distributed around the columnar crystals.Figure 3(e) shows a statistical distribution of grain sizes.With the addition of HEC, the grain size decreased from 3.72 μm to 2.08 μm.The above results indicate that the addition of HEC has the effect of grain refinement.In addition, it is worth noting that in the grain size statistics, when the additions of HEC were 0 and 50 mg l −1 , the statistical deviation values of the grain size were larger, indicating that the grains of ED-Cu exhibited a notable degree of inhomogeneity.When the additions of HEC were 125 and 200 mg l −1 , the statistical deviation values of the grain sizes were smaller, indicating a greater degree of homogeneity in the grains of ED-Cu.This suggests that the addition of HEC was beneficial in enhancing the homogeneity of the grains of ED-Cu.
The dislocation density was evaluated through the analysis of the Kernel Average Misorientation (KAM) and Geometrically Necessary Dislocations (GNDs), as shown in figure 4. Blue areas represent low dislocation density in the region, while green and other colours represent high dislocation density in the region.Figures 4(a)-(d) shows the KAM plots at varying concentrations of HEC.It can be observed that the area of the green region gradually increases with the addition of HEC, from 0 mg l −1 to 125 mg l −1 .The gradual increase in the  dislocation density can be inferred from this observation.Upon the addition of HEC at a concentration of 200 mg l −1 , the area of the green region is found to be smaller than that observed at a concentration of 125 mg l −1 , indicating a decrease in the dislocation density.Compared with the grain boundary distribution features, the blue region corresponded to the columnar crystal region, and the green and other coloured regions were highly overlapped with the fine equiaxial crystal region, which meant that the dislocation density in the columnar crystal region was lower than that in the fine equiaxial crystal region.The average values of KAM and GND were statistically evaluated to assess the dislocation density of ED-Cu along the growth direction at different HEC concentrations, and the results are shown in figure 4(e).The result indicates that the mean values of KAM and GND show a process of increasing and then decreasing, with a turning point in the trend at the HEC concentration of 125 mg l −1 .This indicates that ED-Cu has the highest dislocation density when the HEC concentration is 125 mg l −1 .The areas of higher dislocation densities were mainly located in and near the small grain aggregates, which may be due to the rapid generation of fine grains during the electrodeposition process and the interaction with additives, and the localized stress concentration leading to the accumulation of dislocations.As the grain growth and grain boundary migration happened, and the dislocation density was expected to decrease by recrystallization within the columnar crystals.
Inverse pole figure (IPF) can reveal the preferred orientation of ED-Cu, and the results are shown in figure 5.In the HEC-free condition, the preferred orientation was identified at <111>//Z0 (figure 5(a)).Then it shifted to <001>//Z0 at a concentration of 50 mg l −1 (figure 5(b)).As HEC concentrations continued to increase, further shifts were detected, with the preferred orientation transitioning to <111>//X0, corresponding to concentrations of 125 mg l −1 (figure 5(c)).Upon reaching a concentration of 200 mg l −1 , the preferred orientation became less pronounced (figure 5(d)).The preferred orientations of the three crystal axes showed a higher degree of heterogeneity, demonstrating a heightened intricacy in crystal structure against the further rise in HEC concentrations.When the concentration of HEC increased, the texture intensity firstly increased and then decreased.The texture intensity reached its pinnacle, a value of 3.42, at a HEC concentration of 125 mg l −1  (figure 5(c)).This was the highest point observed during the study.Post this point, the weaving intensity started declining, even while HEC concentrations continued to rise.As the HEC concentration increased to 125 mg l −1 , the texture intensity no longer continued to grow, showing that the orientation of the growth became less preferred.
The influence of HEC on the electrodeposition process of copper was studied by cyclic voltammetry (CV) and the corresponding results are shown in figure 6(a).Figure 6(b) is a partial enlargement of figure 6(a).As shown in figure 6(b), the copper dissolved peak and peak value was the largest in the HEC-free condition, the copper dissolved peak and peak value was significantly reduced after the addition of 50 mg l −1 HEC, which was due to the inhibitory effect of HEC.The dissolved peak area and peak current density were counted, and the results are shown in figures 6(c)-(d).With the increase of HEC concentration, the dissolved peak area of copper was gradually smaller, and the peak current density gradually decreased from 0.1780 ± 0.0030 A cm −2 to 0.1547 ± 0.0097 A cm −2 , indicating that the continued addition of HEC aggravated the inhibition effect.
Chronoamperometry (CA) can provide information about the nucleation and gain growth processes of copper deposition kinetics.CA was used to investigate the nucleation mechanism of copper on the electrode surface at different HEC concentrations, and the results are shown in figure 7. Figure 7(a) shows the current transient's curves, and it can be found that the current density showed a rapid increase in a few seconds, and then slowly levelled off.This was due to the nucleation of copper on the electrode surface during the initial deposition stage, resulting in a rapid increase in current density, which peaks as the number of copper nuclei on the electrode surface increases with the deposition time.As nucleation progresses, the copper nuclei began to overlap.As the copper nuclei formed, there was a diffusion zone associated with the nuclei.Since the diffusion zone was much larger than the nuclei underneath, the overlap zone will eventually cover the entire electrode surface.Further reactions were controlled by the rate of mass transfer through the control region of the diffusion zone [31].In general, the point on the curve where the absolute value of the current density is the highest, which was noted as I m and the time corresponding to the point of greatest current density is noted as t m .As the concentration of HEC increased, I m gradually decreased, and t m gradually increased (figure 7(b)).
The electrodeposition behaviour of copper in the presence of additives can be further understood based on the classic multiple nucleation mechanism under diffusion-controlled growth.According to the Scharifker-Hills (S-H) model [32], the electrodeposition process for copper could be divided into the instantaneous nucleation process and the progressive nucleation process.Figure 7(c) shows the dimensionless current-time curves for copper electrodeposition in electrolytes with different HEC concentrations, as well as the theoretical curves of instantaneous nucleation (blue curves) and progressive nucleation (red curve) of the S-H model.It was found that at the initial stage of electrodeposition (t t m ), the dimensionless curves of different HEC concentrations were close to the instantaneous nucleation curves in the S-H model, indicating that the nucleation mechanism of copper electro-crystallization in this system was a three-dimensional instantaneous nucleation mechanism, and the addition of HEC did not change the nucleation mechanism of copper.When t > t m , the dimensionless curves of different HEC concentrations gradually deviated from the instantaneous nucleation curves [33], which may be due to the reduction of hydrogen on the electrode surface [34].Based on the instantaneous nucleation mechanism in the S-H model, the nucleation density of copper of different HEC concentrations can be calculated.Figure 7(d) shows an enhancing trend with increasing HEC concentrations, where the nucleation density raised from 0.93 × 10 9 /cm 2 to 1.21 × 10 9 /cm 2 , which indicated that the addition of HEC enhanced the nucleation process of copper during electrodeposition.
Figures 8(a), (b) show the tensile stress-strain curves and statistical results of mechanical properties of ED-Cu without pinhole defects prepared with different concentrations of HEC.It is noteworthy that when the HEC concentration was 125 mg l −1 , the mechanical properties showed excellent performance with the highest yield strength (265.10 ± 4.48 MPa), tensile strength (374.88 ± 8.94 MPa) and elongation (7.017 ± 0.775%), indicating that the optimum addition concentration of HEC was 125 mg l −1 .It is worth noting that the trend of yield strength was not consistent with the trend of grain size, but was more consistent with the trend of KAM value (figure 8(c)).This result may be due to the uneven distribution of equiaxed fine crystals at different HEC concentrations, resulting in different effects of dislocation proliferation strengthening.

Causes of defect formation
Several defects appeared while observing the appearance of the ED-Cu.The reason for the scorching area at the top is that this region was at the edge of the plating layer, and the current density became larger due to the current tip effect, while the flow rate of the electrolyte flowing from the bottom to the top was relatively slow in the upper region.As a result, the diffusion rate of the copper ions was slower than the reduction rate in the region, and a hydrogen precipitation reaction occurred at the higher current density, causing the formation of a hydrogen bubble and attached to the reduced deposited copper to form copper powder.This is the reason why the scorching area at the top was observed in all ED-Cu prepared based on VMS with the addition of 0-200 mg l −1 HEC.It is notable that the scorching area at the top of ED-Cu was enlarged when the HEC concentration was raised to 200 mg l −1 , which can be attributed to the polarization of the excess HEC exacerbating the copper deposition inhomogeneity.Pinhole generation is a complex phenomenon.Chloride ions in VMS tend to adsorb  on the microscopically convex part of the substrate [30], while hydrogen ions tend to adsorb on the microscopically concave part of the substrate due to competitive adsorption behavior with chloride ions on the copper surface [35].The presence of chloride ions without HEC accelerated the deposition of copper, leading to a rapid depletion of copper ions in the microregion.This caused a sudden drop in the concentration of copper ions in the local area, while hydrogen ions adsorbed in the depressions could gain electrons and formed hydrogen bubbles.During the deposition process, hydrogen bubbles may not precipitate in a timely manner.This can result in the bubbles occupying the sites of copper reduction, which prevented the reduction of copper ions.As a result, the copper electrodeposition process may cease in localized areas, leading to the formation of pinhole defects.Pinholes and translucent areas are clearly observed in figure 1 both in the HEC-free condition and when a small amount of HEC was added.When the suitable amount of HEC was added, it reduced the surface tension of the electrolyte.This facilitated the immediate discharge of hydrogen bubbles, which helped to avoid the defects of pinhole and translucent areas.

The role of HEC in acid copper plating process
In the electrochemical characterization, HEC has a polarizing effect in the electrolyte and inhibits copper deposition.Further investigation of the nucleation mechanism of copper in the presence of HEC showed that HEC enhanced the nucleation process of copper during electrodeposition.The inhibition of the copper deposition rate and the enhancement of the copper nucleation process resulted in the inhibition of the copper growth process, which in turn led to a decrease in grain size [36].The microstructure analysis revealed that the addition of HEC had a refining effect on the grains.This finding supports the results of the electrochemical analysis.
The mechanism of HEC was determined by combining the analytical characterization results mentioned above.The corresponding mechanism of HEC derived from its adsorption effect.When in the HEC-free condition (0 mg l −1 HEC) (figure 9(a)), The chloride ions present in VMS tended to adsorb onto the protrusions on the metal surface, and then low nucleation rate promoted the rapid growth of grains on the protrusions and induced the formation of coarse columnar crystals.Additionally, the tip effect and the depolarizing effect of chloride ions aggravated the inhomogeneity of copper deposition, the deposition rate of copper in the protrusions was faster than that in the depressions, and the hydrogen bubbles could not be discharged in time after the hydrogen precipitation reaction in the depressions, resulting in the generation of pinhole defects and increasing the surface roughness of the ED-Cu.When adding a suitable HEC (50 and125 mg l −1 HEC) (figure 9(b)), HEC and chloride ions could exert a synergistic effect.Compared to VMS, the addition of suitable HEC has a positive effect on the growth of fine columnar crystals, improves the surface tension of the electrolyte, promotes the desorption of hydrogen bubbles, and suppresses pinhole defects.Additionally, the polarization effect of the added HEC can weaken the depolarization effect of chloride ions, reducing the unevenness of copper deposition and resulting in a lower surface roughness of ED-Cu.When adding an excessive HEC (200 mg l −1 HEC) (figure 9(c)), the copper nucleation process could be enhanced, resulting in a reduction of grain size and a positive effect on the growth of equiaxed grains.Besides, the adsorption of localized excess HEC led to an uneven growth rate of copper, resulting in a rougher surface.

Conclusions
The effects of the HEC concentrations on the ED-Cu with a thickness of 6 μm were investigated based on VMS.It was found that the addition of HEC was beneficial to inhibit the formation of pinhole defects, and the surface of ED-Cu gradually became smooth and then roughened with the increase of HEC concentration.The EBSD results showed that the cross-sectional grain boundary distribution of ED-Cu along the growth direction was a mixed growth of columnar and equiaxed crystals, and the grain size decreased with the increase of HEC concentration.This result was due to the polarization effect of HEC in the electrolyte, and the addition of HEC enhanced the nucleation process of copper and increased the copper nucleation rate, resulting in the reduction of grain size.The dislocation density of columnar crystals was smaller than that of equiaxial crystals, and the high dislocation density area was mainly distributed near the small grains.The addition of HEC caused a change in the preferred orientation of ED-Cu.The copper electrodeposition nucleation could be considered as an instantaneous mechanism.The addition of HEC did not change the nucleation mechanism of the copper electrodeposition process in the system.The optimum concentration of HEC was 125 mg l −1 , and the mechanical properties of ED-Cu were the best at this concentration.
Figure1shows the appearance of the matte and shiny surfaces of the ED-Cu, and the photographs were taken respectively, as shown in figures 1(a), (b).The ED-Cu exhibited a light pink hue overall.However, within the range of 0-200 mg l −1 HEC addition, various appearance defects are observed, including scorching at the top, pinholes, translucent areas, and coloured patches.The scorching at the top was caused by the shedding of copper powder during the cleaning process after plating, and the generation of copper powder is due to the hydrogen reaction[30].Pinholes are small holes with diameters of 30-400 μm that pass through the ED-Cu, which was caused by non-conductive impurities adsorbed on the cathode or by hydrogen bubbles that do not desorb immediately[23].Translucent areas were formed by multiple pinholes for reasons similar to pinhole formation.Colour patches were an area that differs in colour from the surrounding plating and can be attributed to abnormal copper deposition.In the HEC-free condition, the ED-Cu showed pinholes and translucent areas.Interestingly, the pinhole defects showed a significant reduction trend with the gradual addition of HEC, indicating that the addition of HEC on the HEC-free suppressed the pinhole defects.At a HEC concentration of 30-40 mg l −1 , the appearance showed a colour patch.Further addition of HEC concentration to 50 mg l −1 , it was suitable concentration to obtain ED-Cu with basically no defects on the appearance.In addition, it should be noted that the HEC concentration of 200 mg l −1 resulted in further expansion of the deficiencies area of the ED-Cu, which is considered an excessive concentration.Three-dimensional profiles and the micromorphology of ED-Cu matte surface are shown in figure2.Figure2(a) shows the decrease of roughness with the increase of the HEC concentration, which is most pronounced below 40 mg l −1 and then remains in a relatively flat interval.When the HEC concentration exceeded 100 mg l −1 , the roughness began to increase slowly.A sharp decrease in roughness with the addition of 0-40 mg l −1 HEC to the VMS is associated with the elimination of translucent areas and the reduction of pinhole defects.Figures2(b1)-(e1)shows the three-dimensional profiles of ED-Cu at representative concentrations.At 0 mg l −1 HEC (figure 2(b1)), the three-dimensional profile of ED-Cu fluctuated greatly with a peak-to-valley drop of 5.26 μm, which was caused by the presence of many pinhole defects in ED-Cu.When the HEC concentration was 50 mg l −1 (figure 2(c1)), the peak-to-valley falloff of ED-Cu decreased to 3.57 μm.With the increase of HEC concentration from 50 mg l −1 to 125 mg l −1 (figure 2(d1)), the crests and valleys of the ED-Cu surface flattened out, and the peak-to-valley falloff became smaller to 2.9 μm.When the HEC concentration reached 200 mg l −1 (figure 2(e1)), the peaks and valleys of the ED-Cu surface became larger, reaching 3.46 μm.The microscopic surface morphology of ED-Cu are observed, and its SEM images are shown in figures 2(b2)-(e2), with the addition of HEC, the hemispherical copper particles protruding from the surface of ED-Cu become progressively more obvious.Overall, as the HEC concentration increased, the lustreless surface showed a transition pattern from wavy with the condition of HEC-less, to a smoother state with the condition of HEC-suitable, and finally back to wavy with the condition of HEC-excessive.Based on the observation of ED-Cu, four different HEC concentrations were added to VMS, and the effect of HEC on the microstructure of the ED-Cu was studied.EBSD analysis was used to get information such as grain boundary distribution characteristics and orientation.Figure3shows the grain boundary distribution of the

Figure 6 .
Figure 6.CV tests with various HEC concentrations (a) CV curve, (b) local zoom of the electrolyte, (c) dissolved peak area, (d) dissolved peak current density.

Figure 7 .
Figure 7. CA analysis at different HEC concentrations: (a) current transients' curves for copper electrodeposition; (b) statistics of t m and I m ; (c)the dimensionless curves; (d) nucleation density.

Figure 8 .
Figure 8.(a) Stress-strain curves, (b) mechanical properties of ED-Cu with various HEC concentrations; (c) relationship between KAM average and yield strength.