Visualizing electrochemical zinc deposition and the role of a polymer additive in the crystal growth mechanism

Electrodeposition of metals is relevant to many research fields including catalysis, batteries, antifouling, and anticorrosion coatings. Compared with hot dip galvanizing, there is significant interest in less energy and material‐intense electroplating of zinc. At present, large‐scale electroplating mostly uses acidic zinc solutions, containing potentially toxic additives. Alkaline electroplating of zinc offers a route to using environment‐friendly green additives. Further to the previous elucidation of the mechanism by which the polyquarternium polymer (PQ) aids the deposition of negatively charged zincate, here the nature of the coating is explored. Zinc was deposited from an electrolyte including zincate and PQ, on gold model surfaces. Atomic force microscopy (AFM) and low energy ion scattering have been used to characterize the layer and explore the crystal growth mechanism that the PQ changes and, hence, improves the coating quality. We have also used AFM of the PQ on negatively charged mica to provide further evidence for the growth mechanism interpretation. Our data demonstrate that the additive is crucial to steering the growth mechanism, offering routes to optimizing deposition.


| INTRODUCTION
It has been more than a century since Snowdon investigated the properties that influence the electrolytic deposition of zinc. [1]He found that deposition from an alkaline zinc bath is smoother than from an acidic one.He also made one of the first investigations on the use of organic additives (resorcinol) in the zinc bath. [1]Subsequently, electroplating of zinc has become very important for the metal industry.[4][5][6][7] The purpose of using those additives is first to achieve a better working efficiency of the electrolyte, through improvement of the current efficiency or the anodic depolarization.The second purpose is to achieve a better physical appearance and properties of the deposited layer through the improvement of the grain size and the brightness.
Figure 1 shows the influence of an additive particularly clearly with the images of two different zinc layers deposited on a steel sheet.In Figure 1a the zinc layer was deposited from an alkaline zinc bath containing no polymer additive.The deposited layer is matte, rough and has an uneven layer distribution.On the high current density side the deposited layer appears as a fine powderlike structure.In addition, the coating is nonadhesive on the steel part.In Figure 1c polyquarternium (PQ) 2 polymer was included as an additive.With this additive the deposited layer appears smooth, bright and has an even layer distribution.The dramatic difference in the color and homogeneity of the coating is not the only parameters that the additive influences.As is shown further in Figure 1b,d the layer thickness distribution of the zinc on the steel sheet is also changed by the additive.For both sets of conditions from the left to the right side of the sheet, the layer thickness decreases, which arises from the current density gradient created during deposition in a Hull-cell (see Figure 1e).
A Hull-cell [8] is a laboratory-scale version of a standardized electroplating system.It consists of a trapezoidal container made of an insulator.The electrodes are arranged in such a way that cathodic or anodic effects can be observed over wide current density ranges.Bath parameters, such as additive concentrations, pH, temperature, electrolyte composition, and so forth, influence the properties of the deposited layer.The Hull-cell is designed to determine these influences as a function of the current density by varying the distance between the steel cathode and the steel anode.The influence of one such additive, PQ 2 is shown in Figure 1b,d, illustrating the layer thickness distribution without and with the polymer.There is a greater reduction in thickness on the high current density side.
However the ideal case would be to have a constant layer thickness distribution over the whole sheet.Although the polymer improves the deposition a gradient in the layer thickness distribution remains.
Here, we investigate how PQ 2 acts as an additive in an alkaline, cyanide-free electrolyte.How does the polymer moderate the crystal growth of the electrodeposited layer?To answer this question, atomic force microscopy (AFM) was performed to image the deposited zinc layer and so allowed us to visualize the structure of the layer at the nanoscale.AFM is a technique that can be applied to image surfaces, such as metals, [9,10] ceramics, [11,12] polymers, [13,14] and so forth.We additionally used low energy ion scattering (LEIS) to understand the composition of the deposited surface and hence further interpret the surface morphology.Thus, one can have a better understanding of the influence of the polymer on the quality of the coating.By performing AFM of a polymer layer on mica, the conformation and role of the polymer in the changes that it causes to the crystal growth mechanism is supported.

| Chemicals
The zinc oxide (99.7% purity) as well as the NaOH solution (50%) were purchased from VWR International and the PQ 2 (62 wt% in water) from Sigma Aldrich.All chemicals were used as received without further purification.The alkaline noncyanide zinc electrolyte was prepared by dissolving the zinc oxide in NaOH solution and using deionized water to achieve the desired concentration.The concentration in the bath is 3.8 M for the NaOH and 0.2 M for the zinc.The PQ 2 in the electrolyte is adjusted to 0.1 wt%.

| Surface preparations
We used Nordland optical adhesive 81 to attach the sample to the AFM specimen disc.Electrochemically deposited zinc surfaces were prepared as described below before being gently rinsed with water and finally dried with nitrogen before imaging in the AFM.For the experiment with PQ 2 on mica (from the full alkaline, zinc electrolyte as prepared above) we deposited a drop (10 μL) of the solution on a freshly cleaved mica piece (grade V1) and rinsed it off gently with Milli-Q water after approximately 5 min it was then dried at ∘ 60 C before imaging in the AFM.The sample was then kept in a Petri dish at room temperature for 3 days before further imaging.

| Electrochemistry
The zinc layer was deposited at room temperature during chronoamperometry on a gold model surface using a PalmSense4 potentiostat.During the experiments the solutions were bubbled continuously with argon.The working electrode (WE) was a molecularly smooth gold surface prepared by template stripping from 100 nm gold deposited on mica [15] with a geometrical area of 1 cm 2 and an root-mean-square roughness well below 1 nm over several cm 2 .The flat, glass-supported electrode was contacted with a gold wire.Experiments were carried out in a three-electrode setup in a cylindrical PEEK cell with platinum mesh as counter and Ag/AgCl electrode (in 3 M KCl, in a Luggin capillary) as the reference.For each experiment, fresh gold electrodes were stripped shortly before the experiment, and the WE was further electrochemically preconditioned in a 0.1 M NaOH solution by potential cycling (between −0.3 and 0.3 V vs. Ag/AgCl at a rate of 100 mV/s).Each layer was deposited using chronoamperometry at fixed potentials for 30 s.

| Atomic force microscopy
The AFM images were recorded in air at room temperature using an Asylum Research Cypher ES Atomic Force Microscope.In tapping mode we used a silicon cantilever Tap300-G from budget sensors with a resonance frequency of 300 kHz and a nominal force constant of 40 N/m.The volumes of the polymer islands were determined as histograms of grains with a minimum height threshold of 1 nm using the Gwyddion software. [16]

| Low energy ion scattering
High sensitivity-low energy ion scattering (HS-LEIS) spectroscopy measurements were performed using an ION-TOF Qtac 100 spectrometer (IONTOF) with 4 He + at 3 keV as primary ion at an incident angle of ∘ 0 and scattering angle of ∘ 145 .The measurement area was 1.5 × 1.5 mm 2 .The sputter depth profiling was performed with a sputter beam consisting of 40 Ar + 500 eV and 2 keV, respectively, at an incident angle of ∘ 59 and a sputter area of 2.5 × 2.5 mm 2 .The HS-LEIS signals for Zn and Au were quantified against metallic Zn and Au reference standards, with the residual ascribed to the elements H, C, N, O, and Na.An approximation of the depth per sputter cycle was performed via the software ITCalculator (IONTOF) using the respective sputter yields of the elements as well as the measurement parameters.This has resulted in a step size of 0.1-0.15nm for Ar + 500 eV, while no value was determined for Ar + 2 keV due to the large range of variation.

| RESULTS AND DISCUSSION
In this work we used AFM to study the difference between two types of zinc layer.Zinc layers were deposited from an electrolyte containing PQ and from one with no polymer additives.This method allows us to image the morphology of the deposited zinc layer.We will discuss the differences between surface images of the variable layers.This will additionally help us understand why PQ enables the deposition of a thin, smooth, and bright zinc layer.
The imaging reveals some interesting differences.Figure 2 illustrates the results of the imaging of the zinc layer deposited at the potential of −1.48 V versus Ag/ AgCl from an alkaline electrolyte containing no additives.At this potential, the zinc deposition and the hydrogen evolution reaction (HER) happen simultaneously. [17]It should also be noted that this is an earlier onset of the HER, with the bulk Zn deposition, compared with when polymer additives are included in the electrolyte.All the images in the figure were obtained from the same tip position and are represented as height and phase retrace.We can see in all four images that the zinc layer is composed of a variable grain size.Additionally in Figure 2a,b we can clearly see the height variation, which shows that the deposition is uneven.The crystals have different sizes and their shapes are not uniform.The rough surface therefore will not reflect light effectively, as also observed after a significantly longer deposition on the macroscale in Figure 1a.
Although we have previously proposed a mechanism by which the polymer slows the deposition of zinc, [17] here we were interested specifically in what influence the polymer has on the crystal growth, so that it induces the formation of a smooth and bright layer.In comparison to the AFM images of Figure 2 deposited without PQ, Figure 3 shows the substrate surface after the 30 s zinc deposition at −1.43 V versus Ag/AgCl from an alkaline electrolyte containing PQ.At this potential, the positively charged polymer coil approaches the surface and the zincate, that is adsorbed within the polymer coil, is deposited, depleting the polymer. [17]he first observation we make in Figure 3a,b, is that, at first glance, the surface does not appear to be completely covered with zinc.This could be explained by the deposition at this potential having only just started.We hypothesize that we may be imaging the first few deposited monolayers of the zinc deposition, particularly in the center of the image that we image in higher resolution in Figure 3c,d.We will return to this hypothesis after a discussion of the other features observed.In Figure 3b we additionally see that those crystals, which have started to grow, have a similar oval shape.In the height and phase retraces for the 1 μm image (Figure 3c,d) there is no variation in height.However, there are some small variations in the phase retrace Figure 3d, although it is not possible to clearly determine the morphology that they represent.Overall, the images in Figure 3 indicate that when the zincate first deposits from the polymer, it is likely that any crystals growing on the surface of the substrate do not follow the same growth mechanism as seen without the polymer in Figure 2.
To determine whether, as proposed in our above hypothesis, zinc is present across the whole surface even at a deposition potential of −1.43 V, rather than just as islands we performed LEIS spectroscopy.In Figure 4a, the concentration of zinc and gold determined by normalization to a standard is followed, with each spectrum provided in panel Figure 4c.The data points were taken as a sputter depth profile using an Ar + ion sputter gun at two different sputtering energies as indicated in the figure.As highlighted in Figure 4b, after the first sputter cycle no gold is recorded but there is zinc present, followed by an increase of both zinc and gold.Although the background intensity of the spectra and other elemental peaks present indicate that the surface is not purely zinc on gold, the data suggest that the zinc is, likely, well distributed across the surface.This is followed by alloying of zinc with the gold through to approximately 4 nm into the top of the gold surface.Here, all other elements (e.g., organic contaminants and oxygen from oxide formation) are represented by the gray points in the figure.It is interesting to see that there is a significant proportion of other contaminants either present or carried deeper into the surface layer during the LEIS measurement.This can most likely be explained by the incorporation of the polymer and electrolyte species during the crystal growth or from inter-mixing during sputtering.This intermixing could be varied by the sputter settings and, therefore, is likely to represent the highest possible thickness of the alloyed region.
However, to fully understand the ion scattering from the surface alloy of zinc and gold accurately would require a more detailed study of the surface components.When combined with modeling of the ion trajectories and any ion-induced mixing within the modified crystal lattice, LEIS will be able to further elucidate the full structure of the zinc layer formed on Au.
Comparing the topography of the zinc in Figure 2 (deposited without PQ) with the topography in Figure 5 (with PQ) shows a huge difference in the structure.Note that Figure 5 illustrates the imaging of the zinc layer deposited at −1.56 V versus Ag/AgCl from an alkaline electrolyte containing PQ compared with the potential of −1.43 V that was used in Figure 3.Here a diffusioncontrolled zinc deposition happens as this potential represents the start of the main bulk deposition of Zn. [17] At this lower potential, Figure 5a shows less height variation than the zinc layer in Figure 2a.This means that the deposition with PQ leads to a smoother and more even layer.Zooming in at the center of this image shows, as illustrated in Figure 5c,d, that the grains have an elongated shape and approximately the same grain size overall.The surface is smooth and well covered.It is clear that when the crystal growth continues with this mechanism and uniformity, the deposition moderated by the polymer, PQ, will also form a smooth, even, and bright layer also at the macroscopic level as observed in Figure 1.
The way in which the PQ adsorbs at the negatively charged surface and hence directs the crystal growth can be further explored using ex situ images of the PQ adsorbed to a negatively charged mica surface.This approach has the advantage that the mica does not lose its charge while handling the sample. [18]Figure 6 shows circular islands of polymer on the mica surface, which, even after a period of heating, continue to dry out at room temperature and pressure with the radius of the islands continuing to reduce as seen in panels a and c.During repeated imaging these polymer islands could be moved to the edges of the imaging area when the sample was first scanned.However after 3 days of drying, the islands remained stable within the imaging area, indicating that the polymer mobility on the surface was reduced.The number of islands and volume of polymer also increases with the further drying, which indicates that some of the polymer was spread across the surface outside of the islands.The lower resolution of the islands in the height image compared with the phase images in Figure 6b, where they are well defined with a round disc-like shape, could also indicate a covering of polymer across the surface below the islands after the shorter drying period.
In Figure 7 we further analyzed the volume of the polymer islands in a set of AFM images and compared them to the volume distribution that can be estimated for the molecular weight distribution based on the estimated Flory radius, the most likely size characteristic when the polymer is in a good solvent, using , as well as for the R g , which is the more likely characteristic size when dehydrated.The comparison of the size distributions indicates that there is more than one chain in each of the islands and therefore, in hydrated conditions, the polymer is most likely adsorbed more densely than a single monolayer of polymer.Therefore the diffusion barrier, which the polymer expands and spreads across the surface to form in the hydrated (electrolyte) conditions, is expected to provide good coverage of the surface.The zincate retained within the initially adsorbed PQ will also be well distributed across the surface enabling the initial depletion of the polymer to deliver the zinc everywhere.Figure 8 illustrates the proposed growth mechanism in more detail with the schematic of a noncontrolled (Figure 8a) and a polymer-controlled zinc layer deposition (Figure 8b).When there is no polymer present, Figure 8a, we propose that the crystal growth follows a Volmer-Weber-type, island growth mechanism.Multiple nucleation sites are established on the surface.However, further crystal growth continues on top of those initial nucleation sites, inducing the growth of an island.The incorporation of the atoms does not happen uniformly and causes the crystal grains to grow unevenly.They also appear to form new nucleation sites on top of the islands leading to crystals that are inherently weakly bound to the surface.
However, when a polymer additive, PQ, is included in the electrolyte, the crystal grain morphology observed in the AFM images indicates that the mechanism is different.We therefore propose that it proceeds as follows.In step 1 of Figure 8b, the zincate uses the polymer to approach the surface through the inner layer of the electric double layer.As soon as the potential of −1.43 V is reached the polymer is depleted of zinc and the reduction of zinc onto the surface starts as shown in step 2 of Figure 8b.The first layer, or layers of zinc, appears to completely cover the surface with perhaps some islands on top.This is supported by both the AFM images in Figure 3 and the LEIS data presented in Figure 4, which indicates that there is zinc at the surface of the gold through to approximately 4 nm into the top of the gold surface.
In step 3 when the potential for bulk deposition, −1.56 V, has been reached, we see that the surface is completely covered.In contrast to the case where there is no polymer, the grain size is smaller.The crystallites are also smoother and more evenly distributed across the surface.This suggests that the growth mode has changed from the Volmer-Weber-type island growth mechanism to a more layer-by-layer dominated Stranski-Krastanovtype crystal growth mechanism.The fact that the polymer inhibits the deposition and creates a diffusion barrier to the zinc reduction reaction, appears to give time to the zincate to minimize its energy within the growing crystal structures at the surface and thus find a suitable place to deposit.Hence smooth, small, more uniform crystallites are formed compared with the deposition without PQ.

| CONCLUSIONS
It is well known that polymer additives play an important role in forming smooth, visually appealing, shiny zinc layers during electrodeposition.We have previously established that, with the zincate and the substrate surface both negatively charged, the deposition must happen through an outer-sphere reaction. [17]Here we have explored the deposition and growth mechanism during electroplating of zinc from an alkaline zincate electrolyte.Deposition of zinc without a polymer additive led to a rough and dull layer.Including the polymer additive, PQ, improves the appearance of the deposited metallic layer, to be thin, shiny and smooth.The growth mechanism of the zinc crystallites at the model gold surface appears to be governed by the electrostatic charges present at the surface.When there is nothing to slow the reduction of zincate to zinc at the surface, an island (Volmer-Weber-type) growth mechanism dominates.The polymer, PQ, appears to wet the negatively charged surface well, as also indicated in the dried images of PQ on a negatively charged mica surface, it therefore creates a diffusion barrier across the whole surface.Once the positively charged polymer is present to overscreen the charge at the surface, the mechanism appears to change to a layer-by-layer dominated crystal growth mechanism.This leads to smoother, more uniformly sized crystallites.Our further understanding of the growth mechanism of electrochemically deposited zinc can be used to improve deposition in industrially relevant conditions.It is clearly the diffusion through the polymer layer, at different charge/potential values, that dictates the appearance of the zinc layer and the rate at which the deposition takes place.Therefore improvements to additives need to take the polymer structure and charge characteristics into consideration.Changes such as increasing flexibility and the positive charge in the main chain of the polymer can help improve the electrodeposition of zinc.In addition to that, the molecular mass also plays a major role.

F I G U R E 1
Photography of a sheet and its layer thickness distribution showing: (a) a zinc layer deposited without PQ, (c) a zinc layer deposited with PQ and (b, d) the corresponding layer thickness distributions.(e) Picture of a Hull-cell with the cathode and the anode represented.CE, counter electrode; PQ, polyquarternium; WE, working electrode.[Color figure can be viewed at wileyonlinelibrary.com]

F
I G U R E 2 AFM images of a zinc layer deposited from an alkaline electrolyte containing no PQ.(a) Height retrace image at 5 μm scan size.(b) Phase retrace image at 5 μm scan size.(c) Height retrace image at 1 μm scan size.(d) Phase retrace image at 1 μm scan size.AFM, atomic force microscopy; PQ, polyquarternium.F I G U R E 3 AFM images of a zinc layer deposited from an alkaline electrolyte containing PQ at the polymer depletion potential −1.43 V. (a) Height retrace image at 5 μm scan size.(b) Phase retrace image at 5 μm scan size.(c) Height retrace image at 1 μm scan size.(d) Phase retrace image at 1 μm scan size.F I G U R E 4 Low energy ion scattering results for depth profiles of zinc deposited on gold from the electrolyte containing PQ, taken using an Ar + ion sputter gun set at 500 eV (blue shading) and 2 keV (orange shading).In (a) the concentrations have been normalized to pure standards of Zn and Au, where for Ar 500 eV each sputter cycle represents approximately 0.1-0.15nm of depth into the sample.The red box indicates the area shown in detail in (b).(c) The raw spectra for a series of sputter cycles with contributions from elements other than Au falling away as pure gold is reached after 38 cycles.PQ, polyquarternium.[Color figure can be viewed at wileyonlinelibrary.com]

F
I G U R E 5 AFM images of a zinc layer deposited from an alkaline electrolyte containing PQ for the bulk deposition (−1.56 V).(a) Height retrace image at 5 μm scan size.(b) Phase retrace image at 5 μm scan size.(c) Height retrace image at 1 μm scan size.(d) Phase retrace image at 1 μm scan size.AFM, atomic force microscopy; PQ, polyquarternium.

F
I G U R E 6 AFM images of the adsorbed PQ on mica which was deposited from the zinc electrolyte, rinsed, and dried for (a, b) 10 min at ∘ 60 and (c, d) a further 3 days at room temperature.(a, c) The respective height retrace images over a 5 μm scan area and (b, d) the phase retrace images.AFM, atomic force microscopy; PQ, polyquarternium.

FF 3 5
I G U R E 7 Estimated RMS radius of gyration R g for the measured M W distribution using a freely jointed chain model and an estimated Flory radius using ≈ ∕ R ln in purple for the same distribution as the black line.Histograms from the 5 μm height AFM images of 10 min drying (black) and 3 days drying (green).PQ was deposited on mica from a basic solution of PQ.AFM, atomic force microscopy; PQ, polyquarternium; RMS, root-mean-square.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 8 Model sketch illustrating the zinc deposition in (a) an electrolyte containing no polymer and (b) an electrolyte containing polyquarternium.[Color figure can be viewed at wileyonlinelibrary.com]