Effect of Structure of Organic Additives on Electrodeposition Behavior of Zn from Alkaline Zincate Solution and Its Crystal Morphology

Abstract

The effect of structure of organic additives on the electrodeposition behavior of Zn from alkaline zincate solution and its crystal morphology was investigated. Zn was electrodeposited on an Fe electrode at 20–1000 A·m⁻², 2.4 × 10⁴ C·m⁻², 300 K from unagitated zincate solutions containing the various organic additives as a leveling agent. The suppression effect of additives on the charge transfer and diffusion of ZnO₂²⁻ ions in Zn electrodeposition corresponded to the number of adsorption site per a straight chain molecule of polymer. The effect of polymer alone on the decrease in size of Zn platelets crystals was small, but the crystal size significantly decreased with coexistence of low molecular additive. The crystal size of deposited Zn decreased in spite of small suppression effect on Zn deposition, showing that the crystal size of deposited Zn doesn’t depend on the overpotential for deposition. With coexistence of low molecular additive with polymer, the crystal of deposited Zn was fine regardless of kind of polymer even though Zn deposited at the diffusion control of ZnO₂²⁻ ions.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 88 (2024) 58–67.

Fig. 11 SEM images of Zn films deposited at 200 A·m⁻² from the zincate solutions containing both additives of polymer and low molecular compound. [(a) Q, (b) PB + Q, (c) PM + Q, (d) PB·PM + Q]

1. Introduction

Zn electroplating has been performed using solutions such as zincate [1–6], borofluoride [7], chloride [8–11], sulfate [12–16], pyrophosphate [17, 18], and cyanide [19, 20]. The throwing power, appearance, hardness, overvoltage, and current efficiency of plating films depend on the type of solution. Alkaline electrolytes are known to provide excellent throwing power. Recently, alkaline zincate solutions, whose composition is simple and easy to control, have been widely used.

In Zn electrodeposition from a zincate solution, it is essential to use additives to obtain a smooth surface, and many studies have been performed on the effect of additives on Zn deposition behavior and crystal morphology [21–24]. For example, Zn deposition was suppressed with a quaternary ammonium salt, and its suppression effect increased with the presence of a nicotinic acid sulfonate salt to obtain a smooth appearance [21]. Zn deposition was suppressed by polyquaternium-2, and its suppression effect was further increased in the presence of sodium propargyl sulfonate to produce glossy films composed of fine crystals [22]. In contrast, glossy films with a preferred orientation of {11$\bar{2}$0} were reportedly produced by the addition of the reaction products of hexamethylenetetramine and epichlorohydrin [24].

As mentioned above, although Zn deposition from a zincate solution containing additives has been reported, there are many uncertainties regarding the effect of the structure of additives on Zn deposition behavior and the structure of the deposited films. Straight-chain polymers containing quaternary ammonium ions have been reported to act as leveling agents in Zn plating from zincate solutions [25–28]. Therefore, in this study, with respect to straight-chain polymers containing quaternary ammonium ions, we investigated the effect of the number of quaternary ammonium ions acting as adsorption sites, the molecular weight of the polymers, and the type of alkyl group in the straight-chain polymers. Their effect on the Zn deposition behavior and morphology of the deposited films was analyzed. It has been reported that deposited films are smoother in the presence of a low-molecular-weight quaternary ammonium salt containing a benzene ring with a straight-chain polymer than with a straight-chain polymer alone; however, the details of the synergistic effect of the low-molecular-weight additives and polymers are unknown. Therefore, the Zn deposition behavior and morphology of the deposited films were investigated in solutions containing both straight-chain polymers whose structure was varied and a low-molecular-weight quaternary ammonium salt. The effect of additives on the Zn deposition behavior is discussed based on the partial polarization curve for Zn deposition.

2. Experimental

The electrolytic solution was produced by dissolving reagent-grade ZnO (0.153 mol·dm⁻³) and NaOH (3.0 mol·dm⁻³) in distilled and deionized water at room temperature. Various additives were added to these solutions.

Figure 1 shows the structural formula of the additives, and Table 1 outlines the additives. In the polymer denoted as PB, the straight-chain alkyl group is a propyl group, with three carbons between each nitrogen atom, and two quaternary ammonium ions, which act as adsorption sites, are on either side of the carbonyl group (–C(=O)–). In the polymer denoted as PM, the straight-chain alkyl group is also a propyl group, but there is only one quaternary ammonium ion next to the carbonyl group. In this study, polymers with one or two adsorption sites around the carbonyl group are called mono- or bis-type, respectively. In the polymer denoted as PB·PM, the end of a PB molecule is connected to a PM molecule. In the polymer denoted as EB, the straight-chain alkyl group is an ethyl group with two quaternary ammonium ions on either side of the carbonyl group. In the polymer denoted as EM, the straight-chain alkyl group is an ethyl group, but there is only one quaternary ammonium ion next to the carbonyl group. The polymers denoted PB5 and EB5 have five times the weight-average molecular weights of PB and EB, respectively. Q is a low-molecular-weight quaternary ammonium salt containing a benzene ring. The polymer and Q were added into an electrolytic solution at concentrations of 1.6 and 0.093 g·dm⁻³, respectively.

Fig. 1

Abbreviation and structural formulae of each additive used in this study.

Table 1 Outline of each additive used in this study.

Electrolysis was conducted using a constant-current electrolysis method without stirring at a current density of 20–1000 A·m⁻², under the application of 2.4 × 10⁵ C·m⁻² of electricity and a solution temperature of 300 K. The amount of electricity of 2.4 × 10⁵ C·m⁻² corresponds to a film thickness of 11.4 µm, assuming the deposition of pure Zn at a current efficiency of 100%. An Fe plate (2.5 cm × 2 cm) was used as the cathode, and an Fe plate (2.5 cm × 6 cm) was used as the anode. The deposited films were dissolved in nitric acid, and the current efficiency of Zn deposition was calculated by determining the quantity of Zn using inductively-coupled plasma optical emission spectroscopy (ICP-OES). The partial current density of the Zn deposition was calculated by multiplying the total current density by the Zn current efficiency (%)/100. When the polarization curve was measured, an Ag/AgCl electrode (saturated KCl, 0.199 V vs. normal hydrogen electrode (NHE), 298 K) was used as the reference electrode; however, the potentials are presented in terms of the standard hydrogen electrode.

In some experiments, polarization curves were obtained using linear sweep voltammetry (LSV). In LSV, a Zn plate (2.5 cm × 2 cm) was used as the cathode and an Fe plate (2.5 cm × 6 cm) was used as the anode, and the current density for the cathodic reaction was measured from an immersion potential to −1.8 V using the potential sweep method at 10 mV·s⁻¹.

The surface morphology of the deposited films was observed using scanning electron microscopy (SEM).

3. Results

3.1 Suppression effect of additives on Zn deposition

Figure 2 shows the polarization curves measured using LSV. The polarization curve (①) obtained from the additive-free solution rises at −1.27 V, and the current density shows a peak above 300 A·dm⁻². The current density thereafter decreases once, then increases again at a potential less noble than −1.5 V. The peak current density (at −1.3 V) is attributed to diffusion control of the ZnO₂² ions. The increase in current density at a potential less noble than −1.5 V may be due to hydrogen evolution. With the addition of the low-molecular-weight Q (②), the current density peak for diffusion-limited ZnO₂² ions decreases somewhat, but shows almost the same trend as the additive-free solution. On the contrary, with the addition of the polymers PB, PM, PB·PM, EB, and PB5 (③–⑦), the polarization curve is greatly polarized at the middle stage of the first rise in the polarization curve; thereafter, the current density peaks once at −1.40 to −1.45 V, and increases again at a potential less noble than −1.6 V. The polarization in the middle of the first increase in the polarization curve is attributed to the suppression of Zn deposition. The current density peak values at the −1.40 to −1.45 V for all polymers (③–⑦) are lower than those near −1.3 V for the additive-free solution. With the addition of PB, the current density peak is the lowest in value. The current density peaks at −1.40 to −1.45 V are attributed to diffusion control of ZnO₂² ions, and the increase in current density at a potential less noble than −1.6 V appears to be caused by hydrogen evolution. The effects of all the additives on Zn deposition were compared using the partial polarization curves for Zn deposition measured by the steady-state method.

Fig. 2

Polarization curves obtained by LSV in zincate solutions containing various additives.

Figure 3 shows the total and partial polarization curves for Zn deposition obtained from solutions containing PB, PM, and PB·PM, whose number of adsorption sites per straight-chain molecule differs. In the solutions additive-free and containing Q alone, because the surface roughness of the deposited films was high, as shown in Figs. 10(a) and 11(a), the correct polarization curve could not be obtained by the steady-state method. As found from the partial polarization curve of Zn (Fig. 3(b)), Zn deposition begins at a potential nobler than −1.4 V, and at the potential region; that is, at the rate-determining region of charge transfer, the polarization follows the order PB > (PB·PM) > (PM). At the potential region less noble than −1.4 V, the partial current density of Zn is almost constant, indicating that the rate-determining step of Zn deposition is the diffusion of ZnO₂²⁻ ions. Comparing the current density of Zn deposition at the diffusion control region of ZnO₂²⁻ ions, the current density follows the order PB < PB·PM < PM. At both rate-determining regions of charge transfer and diffusion, the suppression effect of additives on the Zn deposition follows the order: PB > PB·PM > PM. This order correlates with the number of quaternary ammonium ions per straight-chain molecule or the number of adsorption sites.

Fig. 3

Polarization curves for Zn deposition from the zincate solutions containing additives with adsorption site of different number. [(a) Total polarization curves and (b) partial polarization curves of Zn]

Figure 4 shows the total and partial polarization curves for Zn deposition obtained from solutions containing PB, EB, PM, and EM polymers, in which the alkyl group and the number of adsorption sites on the straight chain varies. The partial polarization curves in Fig. 4(b) show that, at both rate-determining regions of charge transfer (nobler than −1.4 V) and diffusion limitation of ZnO₂²⁻ ions (less noble than −1.4 V), the polarization is larger with PB and EB than with PM and EM. Similarly, the current density of the total polarization curves decreases more with the addition of PB and EB, with more adsorption sites, than with PM and EM, regardless of whether the straight chain is propyl or ethyl (Fig. 4(a)), indicating that the number of adsorption sites is more important than the type of alkyl group.

Fig. 4

Polarization curves for Zn deposition from the zincate solutions containing additives with propyl and ethyl groups. [(a) Total polarization curves and (b) partial polarization curves of Zn]

Then, focusing on the difference between propyl and ethyl groups, at the rate-determining region of charge transfer nobler than −1.4 V, the polarization is slightly larger with PB and PM than with EB and EM, but the difference is small (Fig. 4(b)). At the diffusion control region of ZnO₂²⁻ ions less noble than −1.4 V, there is little difference in the current density between PB and EB, or between PM and EM.

Figure 5 shows the total and partial polarization curves for Zn deposition obtained from solutions containing PB, PB5, EB, and EB5 polymers with different molecular weights. As can be seen from the partial polarization curves of Zn (Fig. 5(b)), at the rate-determining region of charge transfer nobler than −1.4 V, the polarization is smaller for PB5 and EB5 than for their lower-molecular-weight analogs PB and EB. At the diffusion control region of ZnO₂²⁻ ions, the current density is higher for EB5 than for its lower-molecular-weight analog EB, but there is little molecular weight effect between the PB and PB5.

Fig. 5

Polarization curves for Zn deposition from the zincate solutions containing additives of different molecular weight. [(a) Total polarization curves and (b) partial polarization curves of Zn]

Figure 6 shows the total and partial polarization curves for Zn deposition obtained from solutions containing PB and PM as well as Q. Figure 6(b) shows that, at the rate-determining region of charge transfer nobler than −1.4 V, the polarization does not differ significantly between PB and PM with or without Q. At the diffusion control region of ZnO₂²⁻ ions less noble than −1.4 V, however, the current density decreases slightly in solutions with the presence of Q.

Fig. 6

Polarization curves for Zn deposition from the zincate solutions containing both additives of polymer and low molecular compound. [(a) Total polarization curves and (b) partial polarization curves of Zn]

Figure 7 shows the total and partial polarization curves for Zn deposition obtained from solutions containing PB and PB5 polymers with Q. There is little change in the partial polarization curve in the solution containing PB with Q, but the suppression effect of PB5 increases in the presence of Q (Fig. 7(b)). This indicates that the synergistic effect of the polymer and Q increases with increasing polymer molecular weight.

Fig. 7

Polarization curves for Zn deposition from the zincate solutions containing both additives of polymer and low molecular compound. [(a) Total polarization curves and (b) partial polarization curves of Zn]

Figure 8 shows the total and partial polarization curves for Zn deposition obtained from solutions containing PB, PM, and PB·PM with Q. The same deposition trend is observed as that without Q (Fig. 3), and the suppression effect of the polymers follows the order: (PB) > (PB·PM) > (PM). This order corresponds to the number of adsorption sites per straight-chain molecule.

Fig. 8

Polarization curves for Zn deposition from the zincate solutions containing both additives of polymer and low molecular compound. [(a) Total polarization curves and (b) partial polarization curves of Zn]

3.2 Effect of additives on the current efficiency for Zn deposition

Figure 9 shows the current efficiency for Zn deposition from solutions containing various additives. In solutions containing PB, PM, and PB·PM, the current efficiency decreases with increasing current density regardless of polymer additive (Fig. 9(a)). At a current density above 200 A·m⁻², the current efficiency follows the order PM > PB·PM > PB. This order is opposite to that of the suppression effect of additives on Zn deposition. In other words, the current efficiency for Zn deposition decreases with an increase in the suppression effect of the additives on Zn deposition.

Fig. 9

Current efficiency for Zn deposition in zincate solutions containing various additives. (Effect of (a) number of adsorption site, (b) kind of alkyl group on the current efficiency)

For PB, EB, PM, and EM, in which the alkyl group and the number of adsorption sites vary, there is little difference in current efficiency between PB and EB or between PM and EM (Fig. 9(b)), indicating little difference between ethyl and propyl groups. This is similar to the finding for the suppression effect mentioned above (Fig. 4(b)). In contrast, the current efficiencies are lower for PB and EB than for PM and EM, regardless of the presence of ethyl or propyl groups. In other words, the suppression effect of the additives on Zn deposition is larger for the bis-type than for the mono-type (Fig. 4(b)).

In solutions containing polymers of PB, PB5, EB, and EB5, there is little difference in current efficiency between EB and EB5 or between PB and PB5. Nor is there a significant difference in current efficiency between PB alone and PB + Q.

3.3 Effect of additives on the surface morphology of deposited Zn

Figure 10 shows surface SEM images of Zn films deposited from solutions containing PB, PM, and PB·PM. The Zn films deposited from the additive-free solution are non-uniformly sized with thick platelet crystals perpendicular to the substrate and a rough surface morphology (a). With the addition of PB, PM, and PB·PM ((b), (c), and (d), respectively), the deposited Zn films become thin platelet crystals perpendicular to the substrate.

Fig. 10

SEM images of Zn films deposited at 200 A·m⁻² from the zincate solutions containing additives with adsorption site of different number. [(a) Additive-free, (b) PB, (c) PM, (d) PB·PM]

Figure 11 shows surface SEM images of Zn films deposited from solutions containing PB, PM, and PB·PM with Q. Zn films deposited from a solution containing only Q have a rough surface composed of massive crystals perpendicular to the substrate (a), while in solutions containing PB, PM and PB·PM as well as Q, the crystals of deposited Zn become fine and the surface is smooth ((b), (c), (d)). The surface is smoothest for PB·PM + Q (d), and the next smoothest is PB + Q (b). Compared with PB, PM, and PB·PM only (Fig. 10), the addition of Q causes the crystals of deposited Zn to become finer and smoother (Fig. 11). In this study, for all straight-chain polymers, the addition of polymers only shows the same trend as that shown in Fig. 10, and the Zn crystals do not become fine without the addition of Q.

Fig. 11

SEM images of Zn films deposited at 200 A·m⁻² from the zincate solutions containing both additives of polymer and low molecular compound. [(a) Q, (b) PB + Q, (c) PM + Q, (d) PB·PM + Q]

Figure 12 shows surface SEM images of Zn films deposited from solutions containing PB, EB, PM, and EM with Q. Comparing PB + Q with EB + Q, both have fine crystals and there is little difference between propyl and ethyl groups ((a), (b)). In solutions containing PM and EM with Q ((c), (d)), the crystals are not as fine as those for PB + Q and EB + Q ((a), (b)). Comparing PM + Q with EM + Q, the surface roughness is slightly higher for EM than PM, but the difference is not large.

Fig. 12

SEM images of Zn films deposited at 200 A·m⁻² from the zincate solutions containing additives with propyl and ethyl groups. [(a) PB + Q, (b) EB + Q, (c) PM + Q, (d) EM + Q]

Figure 13 shows surface SEM images of Zn films deposited from solutions containing PB and PB5 with Q. At a current density of 200 A·m⁻², the crystals for both PB + Q and PB5 + Q become fine and there is little difference in morphology ((c), (d)), while at 100 A·m⁻², the crystals become somewhat larger for PB + Q (a). In other words, at a lower current density, the crystal size is smaller for the polymer with the higher molecular weight.

Fig. 13

SEM images of Zn films deposited at 100 and 200 A·m⁻² from the zincate solutions containing additives of different molecular weight. [(a) PB + Q, 100 A·m⁻², (b) PB5 + Q, 100 A·m⁻², (c) PB + Q, 200 A·m⁻², (d) PB5 + Q, 200 A·m⁻²]

Figure 14 shows surface SEM images of Zn films deposited at various current densities in a solution containing PB and Q. The Zn films deposited at 100 A·m⁻² are composed of platelet crystals approximately 1 µm in length perpendicular to the substrate (a), while at higher current densities of 150 and 200 A·m⁻², the crystals become finer ((b), (c)). With a further increase in current density to 250 A·m⁻², the crystals become somewhat larger, and the surface roughness increases (d). With the addition of PM and Q, the same trend as PB + Q is observed; however, because PB has a more adsorption sites, the crystals are finer and the surface is smoother at all current densities.

Fig. 14

SEM images of Zn films deposited at various current densities from the zincate solution containing additives of PB and Q. [(a) 100 A·m⁻², (b) 150 A·m⁻², (c) 200 A·m⁻², (d) 250 A·m⁻²]

Figure 15 shows surface SEM images of Zn films deposited at various current densities in solution containing PB·PM and Q. The Zn films deposited at 100 A·m⁻² show clear platelet crystals approximately 1 µm in size (a), while at 150 A·m⁻², the crystals become finer and the surface is smoother (b). With an increase in current density to 200 A·m⁻², the crystals become much more fine, resulting in the smoothest films surface (c). At 250 A·m⁻², the crystals become larger and the surface roughness increases (d).

Fig. 15

SEM images of Zn films deposited at various current densities from the zincate solution containing additives of PB·PM and Q. [(a) 100 A·m⁻², (b) 150 A·m⁻², (c) 200 A·m⁻², (d) 250 A·m⁻²]

3.4 Effect of additives on the appearance of deposited Zn

Figure 16 shows the appearance of Zn films deposited at various current densities in solution containing PB·PM with and without Q. The word “nine” written in Chinese characters, seen on the lower left side of the photograph, reflects the word written on a card placed opposite the sample. When the Chinese characters are clearly observable, it can be inferred that the deposited film exhibits high gloss. In additive-free solution, the films deposited at 100 A·m⁻² exhibit a black color, and those obtained at current densities above 150 A·m⁻² become white-gray and are rough. In a solution containing Q only, the films deposited at 150 A·m⁻² become whiter, and those obtained at current densities above 200 A·m⁻² exhibit the same appearance as that for the additive-free solution. In a solution containing PB·PM only, a smooth black-gray surface was obtained, but gloss was not obtained. On the contrary, when both PB·PM and Q were added, the film deposited at 100 A·m⁻² exhibited some gloss, and those obtained at current densities above 150 A·m⁻² showed significant gloss. There is little difference in the gloss of the deposited films between 150 and 300 A·m⁻².

Fig. 16

Appearance of Zn films deposited at various current densities from the solutions containing various additives.

4. Discussion

Table 2 summarizes the effects of the different polymers with varying structure and with and without Q on the Zn deposition. The suppression effect on Zn deposition and the crystal size reduction effect are summarized at both the rate-determining regions of charge transfer and diffusion of ZnO₂²⁻ ions during Zn deposition.

Table 2 Summary of the effect of additives on the Zn deposition behavior and its smoothness.

4.1 Effect of structure of additives on Zn deposition

The polymer additives used in this study suppressed not only the charge transfer of Zn deposition but also the diffusion of ZnO₂²⁻ ions. The suppression effect of additives on the charge transfer of Zn deposition correlates with the diffusion of ZnO₂²⁻ ions; there are some exceptions, but generally, additives that significantly suppress the charge transfer also significantly suppress the diffusion of ZnO₂²⁻ ions (Fig. 3(b)). The diffusion of ZnO₂²⁻ ions is suppressed because of an increase in the viscosity of the electrolyte in the cathode layer, including the diffusion layer, when the adsorption ability of the additives increases or the concentration of additives in the cathode layer increases.

Focusing on the structural features of the polymer additives, the number of quaternary ammonium ions or adsorption sites has the largest effect. As mentioned above, the suppression effect of additives on both charge transfer and diffusion followed the order: (PB) > (PB·PM) > (PM) (Fig. 3(b)), and this order corresponds to the order of the number of adsorption sites per straight-chain molecule. This is attributed to an increase in the adsorption ability of the additives.

Regarding the effect of the molecular weight of the polymer additives, when comparing PB with PB5, the suppression effect on charge transfer was smaller with the higher-molecular-weight PB5, but there was little difference between PB and PB5 on the diffusion of ZnO₂²⁻ (Fig. 5(b)). Assuming that the effect of the end of the polymer on adsorption is large, in the case of an increase in the degree of polymerization, such as PB5, the suppression effect on charge transfer seems to decrease owing to a decrease in the number of molecular ends; however, further investigation is required regarding the effect of the end of the polymer. In contrast, in Zn and Cu deposition from a sulfate solution, the relationship between the molecular weight of polyethylene glycol and gelatin of the straight-chain polymer and its suppression effect on the deposition has been reported. With an increase in molecular weight, the adsorption ability of the polymer or the suppression effect on the deposition was reported to increase owing to an increase in the number of adsorption sites per straight-chain molecule; however, when the molecular weight was greatly increased, the suppression effect was reported to be saturated or decreased due to a decrease in the effective number of adsorption sites because of entanglement of the polymer itself [29–31]. In this study, the suppression effect of the polymer did not increase despite an increase in molecular weight, which is attributed to saturation of the suppression effect owing to excess molecular weight.

Regarding the effect of the type of alkyl group in the polymer additives, when EB and PB were compared, the suppression effect for both charge transfer and diffusion was slightly larger for PB than for EB, but the difference was small (Fig. 4(b)). With a surfactant containing a quaternary ammonium ion, Zn deposition from an acid solution is reportedly suppressed with an increase in the length of the alkyl group [32]; however, with ethyl and propyl groups in this study, the difference in the suppression effect on Zn deposition is small because of a difference of only one carbon in the length of the alkyl group.

In solutions containing PB, PB5, and PM, the current density in the diffusion layer slightly decreased in the presence of Q, indicating the synergistic effect of the polymer and the low-molecular-weight additive (Figs. 6(b) and 7(b)). With the high molecular weight polymer PB5 coexisting with the low-molecular-weight Q, the decrease in current density in the diffusion control region was larger due to a synergistic effect. The reason for this synergistic effect has not been reported, and the details are unknown. The polymers have multiple adsorption sites and are adsorbed on the large surface area of the cathode, whereas the Q seems to be adsorbed on the local site. Therefore, Q could be adsorbed on gaps in the adsorption sites of the polymer, resulting in an increase in coverage of the additives. In this study, the synergistic effect of the additives was largest for PB5 + Q, which is attributed to the gap in adsorption sites of the polymer being the largest. Assuming the coverage of the additive or the concentration of the additive on the cathode increases, suppression of the diffusion of ZnO₂²⁻ ions in the diffusion layer can be explained.

The current efficiency for Zn deposition was generally lower in the presence of additives that suppress Zn deposition. The additives that suppressed Zn deposition also suppressed hydrogen evolution, but the decrease in the current efficiency for Zn deposition indicates that Zn deposition was suppressed more than hydrogen evolution.

4.2 Effect of additives on the morphology of deposited Zn

Although the effect of polymers on the size of the Zn platelet crystals is small, the crystal size significantly decreased in the presence of Q. Based on the overpotential theory for metal deposition, when the overpotential for metal deposition (the difference between the equilibrium potential and the actual deposition potential) becomes large, the rate of nucleation is faster than the rate of crystal growth, resulting in a decrease in the crystal size [33–38]. That is, the overpotential for deposition increases with an increase in suppression of deposition by additives, and the crystals become smaller. In this study, for solutions containing polymers, the degree of increase in the overpotential in the presence of Q was small. Therefore, the observed reduction in the crystal size of the deposited films with the presence of Q cannot be explained by the overpotential theory.

In solutions containing polymers of PB, PM, and PB·PM, the crystal size in the presence of Q followed the order: PB·PM < PB < PM (Fig. 11), and this order does not correspond to that of the suppression effect on deposition: PB > PB·PM > PM. With PM + Q, since the suppression effect on the Zn deposition is smaller than that of the others (Fig. 8(b)), and the crystal size was larger than for PB + Q and PB·PM + Q (Fig. 11). For PB·PM + Q, although the suppression effect was smaller than that with PB + Q (Fig. 8(b)), the crystal size was smaller (Figs. 14, 15). This indicates that the crystal size did not depend on the overpotential, as mentioned above. The structure of the end site of PB·PM is different from that of PB (Fig. 1), and the structure of the molecular end may affect the crystal size of the deposited films. Concerning the deposition behavior, with the presence of both the polymers and Q, the coverage of additives on the cathode is assumed to increase. With an increase in the coverage of additives, nuclear growth appears to be more uniformly suppressed. The change in the crystal size may depend on the covering behavior of the additives.

The negligible effect of molecular weight and alkyl group on the morphology of deposited Zn (Figs. 12, 13) corresponds to the fact that there was little difference in suppression of deposition (Figs. 4(b), 5(b)), and the coverage of additives on the cathode is assumed not to have changed.

With the addition of PB·PM + Q and PB + Q, it is of interest to discuss the relationship between the rate-determining step of Zn deposition and the crystal size of the deposited Zn films. With the addition of PB·PM + Q, the deposition at 100, 150, and 200 A·m⁻² proceeds under the rate-determining step of charge transfer, while at 250 A·m⁻², it proceeds under diffusion control of ZnO₂²⁻ ions (Fig. 8(a)). The crystal size significantly decreased at 200 A·m⁻² (Fig. 15(c)), showing that the crystal size decreased with increasing current density or overpotential for deposition at the rate-determining region of charge transfer. This can be explained by the overpotential theory of deposition. At 250 A·m⁻², where Zn deposition proceeds under diffusion control, the crystal size becomes slightly larger (Fig. 15(d)), showing that the crystal size is smaller at the rate-determining region of charge transfer than that under diffusion control of ZnO₂²⁻ ions.

On the contrary, for the addition of PB + Q, the deposition at 100 A·m⁻² proceeds under the rate-determining step of charge transfer, while at 150, 200, and 250 A·m⁻², it proceeds under diffusion control of ZnO₂²⁻ ions (Fig. 8(a)). The crystal size was smaller at 150 and 200 A·m⁻²; that is, the crystal size was smaller under diffusion control than that at charge transfer (Fig. 14), showing a different trend from that with PB·PM + Q.

In general, when metals are deposited from additive-free solutions under diffusion control, the surface roughness increases due to a decrease in the concentration of metal ions at the cathode layer [33], but with the addition of PB + Q, the crystal size decreased despite the presence of diffusion control of ZnO₂²⁻ ions. With the additives, when the diffusion of ZnO₂²⁻ ions in solution is suppressed, crystal growth of the deposited film is suppressed and the nucleation rate becomes faster than crystal growth, resulting in a decrease in crystal size.

As mentioned above, for both the charge transfer and diffusion, the region where the crystal size became smallest differed according to the type of additive, but in solutions containing polymers, when Q was present, the crystals were fine despite the diffusion control of ZnO₂²⁻ ions.

5. Conclusions

The Zn deposition behavior from alkaline zincate solution (0.153 mol·dm⁻³ ZnO, 3.0 mol·dm⁻³ NaOH, 300 K) containing various kinds of additives and its crystal morphology were investigated. The suppression effect of the additives on the charge transfer and diffusion of ZnO₂²⁻ ions during Zn electrodeposition significantly depended on the number of adsorption sites per straight-chain molecule of the polymers. This is attributed to an increase in adsorption with an increasing number of adsorption sites. With respect to the effect of molecular weight, for propyl-type polymers, the suppression effect on charge transfer was smaller with higher molecular weight, whereas there was little effect of molecular weight on diffusion. With regard to the effect of ethyl and propyl groups in the straight-chain polymers, the suppression effect on the charge transfer and diffusion was slightly larger for propyl groups, but the difference was small.

Regarding the morphology of the deposited Zn films, the effect of the polymers on the crystal size was small. However, in the presence of the low-molecular-weight additive Q, the crystals became significantly finer despite a small effect on polarization for Zn deposition. This indicates that the crystal size of the deposited films does not depend solely on the deposition overpotential. There was little effect of the polymer molecular weight or alkyl group type on the morphology of the deposited Zn films. For both the rate-determining steps of charge transfer and diffusion, the region where the crystal size became smallest differed according to the type of additive. However, in all solutions containing polymer, when Q was present, the crystals were fine, despite diffusion control of the ZnO₂²⁻ ions.

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