3.1. Electrochemical Characterization
Polarization curves of the electrodeposition of tin from the chloride electrolyte on a rotating disk electrode (RDE) are depicted for various ABN concentrations (
Figure 3a) and rotation speeds (
Figure 3b). The measured cathodic current density starts to increase at a potential of −700 mV until a plateau, i.e., a limiting current density, is reached at ca. –880 mV in the electrolyte without ABN addition at a rotation speed of 256 rpm. This can be associated with the diffusion-limited reduction of Sn(II). At cathodic potentials higher than −950 mV the current density further increases due to the hydrogen evolution reaction. Addition of ABN (
Figure 3a) caused a suppression of H
2 evolution within the studied potential window and a decrease in the limiting current density (
iL). This behavior indicates that ABN molecules adsorb onto the cathode surface and interfere with the normal metal deposition by blocking the attachment of tin ad-atoms and delaying hydrogen co-evolution.
RDE measurements at higher rotation speeds resulted in rather complex j-U behavior (
Figure 3b). The current density in the hydrogen evolution potential region (negative of ca. −900 mV) exhibits a moderate plateau at 0 rpm. The Sn(II) reduction is not strongly affected by the convection changes. Only at the highest speed of 386 rpm, the hydrogen current density drastically increased indicating also a change of pH in the electrode vicinity (not shown).
Cyclic voltammograms (CVs) of the electrodeposited tin from the chloride based electrolyte are depicted for different ABN concentrations in
Figure 4a and for different cathodic return points in
Figure 4b. The forward cathodic scan exhibits a single current Sn deposition peak (~ −0.7 V) followed by the limiting current density plateau, as reported for the RDE results. The absence of the cross-over of the forward and reverse scan (
Figure 4a,b) shows that no overpotential was required for the nucleation process of Sn on Au to be started and there is no bulk deposition [
15,
19]. In the anodic direction three current peaks were detected (
Figure 4a,b). According to the literature [
20,
21] tin in a slight acidic media undergoes active dissolution and passivation during anodic polarization. The dissolution of tin occurs in two steps:
Šeruga and Metikoš-Huković [
21] studied the oxidation process in citric buffer. With the consideration that most of the dissolute Sn
2+ forms soluble chelate complexes Sn(HCit) in solutions with pH = 5 and 6, they proposed several oxidation mechanisms which take place in two steps. In the initial passivation step, Sn(OH)
2 is generated, and with further anodic polarization insoluble Sn(OH)
4 is created. According to Šeruga and Metikoš-Hukovič [
21], the tin passivation process in weak acidic electrolytes can be explained by the Müller passivation model and the film formation process is under ohmic resistance control. The creation of the passivation layer refers to peak I and II of the CV in
Figure 4. Considering the drop of the anodic current to 0, peak III refers to the dissolution of the bulk Sn layer. This assumption was confirmed by the measurement with two different cathodic return points −0.8 V and −1 V. When the electrode was polarized to the higher cathodic potential, –1 V, more Sn was deposited, and the Sn dissolution peak III was higher in comparison to that at –0.8 V. The passivation peak heights were independent of the cathodic polarization potential (
Figure 4b). The current efficiency was calculated as the ratio between the charges that were used for oxidation and reduction of Sn (
Qox/
Qred). It was estimated to be 85% for both electrolytes without and with the 3.12 mg/L ABN. On the other hand, the amount of charge used for reduction of Sn was lowered by 10% due to the presence of 3.12 mg/L ABN in the electrolyte. This might indicate that the adsorption of ABN on the cathode surface results either in the specific blocking of the active sites for Sn reduction or in creating a thin layer of ABN on the cathode.
In order to monitor the current for a longer period of time, current density transients were recorded. Resulting
j–
t transients detected for the potential −0.8 V are different in the electrolytes without presence of ABN and with 3.12 mg/L ABN (
Figure 5). When no ABN is present in the electrolyte, the current density has a value of about −0.44 A/dm
2 in the first 100 s which increases to −0.56 A/dm
2 at 300 s. This slight increase might be caused by the increase of the surface area during the deposition. On the other hand, with the presence of 3.12 mg/L ABN, a much lower current flows at the potential of −0.8 V. This means that the addition of ABN to the electrolyte changes the composition of the Helmholtz double layer. Current exponential decay reaches a plateau of −0.1 A/dm
2 after 100 s of electrodeposition. This indicates the inhibition effect of ABN and that the electrode blocking process reaches equilibrium after 100 s of electrodeposition.
The results from the Tafel measurements confirmed that ABN acts as a highly effective inhibitor in the chloride electrolyte. The presence of 0.5 mg/L of the additive in the electrolyte decreases the exchange current density (
j0) to about 50% of its initial value, i.e., from 7.08 to 3.31 mA/cm
2. At the concentration of 0.78 mg/L, ABN acts as an inhibitor at the surface of the electrode and
j0 is one order of magnitude lower than in the case when no ABN additive is present (
Table 2). A significant decrease of the cathodic slope, from −0.0007 to −0.0042 mV, was detected due to the addition of 0.78 mg/L ABN into the electrolyte. With further increase of the ABN concentration, the cathodic Tafel slope did not change significantly. The high values of the Tafel slopes in the additive-free-electrolyte were described by Martyak and Seefeld [
10] as a diffusion controlled reduction process. Moreover, they observed a change of the reduction process from a diffusion controlled to a kinetically controlled due to the adsorption of additives on the cathode surface and the decrease of the Tafel slope. This would indicate, that in the case of MSA electrolyte, the threshold value of ABN concentration for changing the character of the reduction process could be found in the range between 0.5 and 0.78 mg/L. Parallel shifts in the linear logarithmic part of the Tafel lines with no change in the Tafel slopes were detected with an ABN concentration higher than 0.78 mg/L. Based on the observation from Vračar and Dražić [
22], such a shift indicates the blocking effect of the additive.
Polarization curves of the electrodeposition of tin from the MSA electrolyte on a RDE are depicted for various ABN concentrations (
Figure 6a) and rotation speeds (
Figure 6b). Electrodeposition of tin in this case sets in at a potential of −600 mV vs Ag/AgCl. Due to the higher Sn
2+ concentration and high ion mobility in this electrolyte, a limiting current density plateau was found only in the case of a static electrode at −5 A/dm
2 (
Figure 6b). With the introduction of convection, the current density of Sn deposition is shifted to more positive potentials (
Figure 6b) and the limiting current density plateau is no longer visible. With the addition of ABN, cathodic current is not suppressed as occurs in the chloride based electrolyte (
Figure 6a).
For better understanding of the ABN in the MSA electrolyte, polarization, Tafel measurements and galvanostatic deposition from the additive and citrate free Sn MSA based electrolyte system were performed. The results were compared with the initial MSA electrolyte containing 100g/L of tri-sodium citrate (
Figure 7).
The effect of various organic additives e.g. Tritons and polyglycols, on the kinetics of Sn plating has already been studied by Meibuhr et al. [
23]. They observed an increment of the electrode polarization due to the adsorption of nonionic additive on the surface of the cathode. In our case a 15 mV increase in the electrode polarization was detected due to the presence of only 1.56 mg/L ABN in the citrate free electrolyte (
Figure 7). The polarization was not dependent on the rotation speed of the RDE (not shown). This might indicate that ABN adsorption on the cathode is a kinetically controlled action. The effect of the citrate ions in the MSA electrolyte appears to be rather diverse. It is known, that citrate ions in the MSA electrolyte form several types of complexes with Sn(II) ions [
24,
25]. This might indicate the formation of relatively stable citrate complexes that lead to an increase in the polarization at the higher potentials for the deposition reaction of the active tin species (
Figure 7). On the other hand, the presence of citrate shifts the electrode polarization potential into the anodic direction, which is in contradiction to the character of the complexing agent (
Figure 7).
Cyclic voltammograms of the electrodeposited tin from the MSA electrolyte are depicted for different ABN concentrations in
Figure 8. For the whole ABN concentration range studied, CVs show a single peak in both cathodic and anodic direction only, which is in agreement with Šeruga, Metikoš-Huković [
21]. According to their observation, tin passivation in acidic citrate solutions (pH = 3 and 4) exhibits a single anodic peak and the passivation is possible because tin ions and tin-citrate complexes undergo hydrolysis in solutions with lower pH. In this case, anodic process is under diffusion control. The current efficiency for citrate and ABN free electrolyte, calculated as a ratio between charges used for oxidation and reduction of Sn (
Qoxid/
Qred), was 68%. Due to the presence of citrate in the MSA electrolyte, the current efficiency increased to 77%, confirming multiple effects of the citrate in the electrolyte. The ABN in the studied concentration range has no further effect on the current efficiency.
Results from the Tafel measurements confirm the observation that the ABN additive also acts as an inhibitor in the MSA electrolyte. However, in comparison to the chloride electrolyte, a higher ABN concentration is needed for blocking of the electrode surface. A significant decrease of exchange current density,
j0, which indicates slower electron transfer across the tin-electrolyte interface, is reached with the ABN concentration of 4.37 mg/L.
Table 3 and
Table 4 show that the measured exchange current density strongly depends on the electrolyte composition.
MSA electrolytes are known to be high-speed plating electrolytes [
2,
3] where high current densities are used for fast deposition of tin. However, a tin deposit from an additive- and citrate-free MSA electrolyte obtained with current density −1.5 A/dm
2 was dendritic and did not cover the whole substrate surface homogeneously. This could be explained by dendritic growth theory [
26], where dendrites could grow at low current densities when the reaction has high exchange current density. However, the results from the Tafel measurements show a decrease of
j0 with the addition of 100 g/L tri-sodium citrate into the electrolyte from 51.29 to 25.12 mA/cm
2, respectively (
Table 3 and
Table 4). The values of the cathodic Tafel slopes for both citrate free- and citrate rich-electrolytes, are approximately constant within the studied ABN concentration range, indicating a kinetically controlled reduction. The presence of the citrate in the electrolyte changes the character of the tin coating from dendritic to compact-type. Additionally, it also increases the cathodic current efficiency and shifts the electrode polarization potential into the anodic direction. Those findings indicate, that the citrate, apart from being a complexing agent for Sn(II) ions, might also act as a surfactant. The anodic shift of the polarization potential could be explained by the fact, that the surfactants, in general, decrease the surface energy and with that also the activation energy [
27]. The detailed role of the citrate in this electrolyte is beyond the scope of this publication and should be a subject for further study. Analogous to the polarization measurements results, the inhibition effect of ABN is evident in the citrate-free electrolyte. An additive concentration of 3.12 mg/L slows down the ion transfer at the electrode-electrolyte interface and reduces
j0 to 40.74 mA/cm
2 (
Table 4). The surface morphology changed from tree-like dendrites to compact layers with the addition of ABN.
The influence of the presence of ABN and citrate in the MSA electrolyte on the current density transients for the potential −0.65 V is depicted in
Figure 9. The horizontal line marks the current density of −1.5 A/dm
2 at which the dendritic layer from citrate and ABN free electrolyte was obtained. For the studied potential, there is an exponential increase in current as a function of time, indicating the enlargement of the electrode surface area over time and with the deposition of a dendritic layer. Slight retardation of this process was achieved with the presence of 3.12 mg/L ABN and 100 g/L citrate in the electrolyte.
3.3. XRD Analysis
A comparison of the effect of ABN on the preferred crystal orientation of Sn films was conducted by XRD analysis. The analysis was focused on coatings deposited from both chloride and MSA electrolyte, from a stationary plating solution, and with three different ABN concentrations: 0, 1.56, and 3.12 mg/L. To avoid the detection of the substrate peaks, the analysis was performed with a grazing incidence configuration that allows the information depth of the measurements (limited to ~250 nm from the surface of the sample) to be reduced. Hence, a qualitative analysis of the texture of the samples is provided. 3D plots of the XRD spectra of tin coatings deposited from both chloride and MSA electrolytes are shown in
Figure 11. From the analysis of the XRD spectra, all the Sn films deposited from both electrolytes can be considered as randomly-textured. The X-ray spectra of randomly-textured Sn films are commonly characterized by several minor peaks of high Miller-indexed planes [
33], as in the case of the XRD spectra shown in
Figure 11a,b. Such random-textured Sn films are obtained when the deposition is performed at relatively low current density resulting in a slower film growth rate. Applying high current density favors the deposition of Sn coatings with different texture, commonly along low Miller-indexed planes such as (200) or (220) planes [
33]. Such coatings show weak or no diffraction peaks from higher Miller indexes planes, as also seen in previous studies on Sn deposited from chloride based [
11,
34] and MSA based electrolytes [
28]. However, independently of the ABN addition in both electrolytes, the spectra shown in
Figure 11 are characterized by the same number of peaks. In the X-ray spectra of the sample deposited from the chloride electrolyte with no ABN the (101) diffraction peak has the highest intensity. However, with increasing ABN concentration a constant decrease of the (101) intensity is observed (
Figure 11a). The same trend was found for the samples deposited from the MSA electrolyte (
Figure 11b) but less pronounced. This finding strengthens the hypothesis of ABN acting as an inhibitor in both chloride and MSA electrolytes, but to a different extent. The inhibition effect is more pronounced in the case of chloride based solution where ABN modifies the grain structure and strongly reduces the intensity of the (101) peak. The possible reason for the lower inhibition of ABN in the MSA electrolyte could be, that in the citrate-rich electrolyte there is a competition between the effect of the citrate, as a possible surfactant, which lowers the activation energy [
27], and ABN as an inhibitor, which causes a rise in the activation energy [
22]. Further experiments should be performed to confirm this assumption.