1. Introduction
Layer-by-layer (LbL) deposition is a versatile manufacturing technology that allows for the fabrication of thin films composed of polyelectrolytes, nanoparticles, and clay by utilizing electrostatic attraction (or hydrogen bonding) between the components [
1]. It also allows for control of the thickness and structure of the coating in nanometer scale by simply adjusting the number of layers, concentration, temperature, and pH of the dispersion solution. Moreover, LbL deposition is an environmentally friendly method because it is mostly based on aqueous solutions. Many thin coatings based on LbL deposition are now widely investigated for energy, biological, and environmental applications [
2,
3,
4].
Clay is often used in various organic-inorganic composite materials because of its high mechanical strength and thermal stability, low cost, and availability [
5]. Compared to conventional bulk composite materials composed of polymer and clay, a multilayer coating of clay and polymer deposited by the LbL assembly method is more effective at blocking gas diffusion, flame retardation, and metal corrosion protection [
5,
6]. For example, LbL films of polyvinylamine (PVAm), polyacrylic acid (PAA), polyethylenimine (PEI), and montmorillonite (MMT) were shown to achieve an oxygen permeability as low as 0.009 cm
3/(m
2 day atm) [
7]. The permeability decreased when the number of the coated layers increased and depended on the deposited polymers’ types and order. Another study showed that LbL coating of PEI, PAA, and MMT on copper suppressed the corrosion of the metal in H
2S atmosphere about 1000 times via the measurement of the weight reduction [
8]. Additionally, LbL coatings of polymethylmethacrylate (PMMA) and MMT were shown to be effective at inhibiting the corrosion of iron in a salty environment by using electrochemical impedance microscopy. The degree of corrosion protection depended on the shape of the clay used [
9].
In this paper, an LbL coating composed of MMT and PEI (LbL MMT-PEI) was deposited on metal substrates, and mass transport through the coating was investigated via cyclic voltammograms of the coated electrode in a solution of the redox couple, indigo carmine (IC). The gold electrode coated with LbL MMT-PEI showed a lower redox peak current than the bare electrode, and the current also decreased as the number of coated layers increased. Furthermore, the blocking efficiency of the LbL coating was varied by the structure, and the lower redox current of the coated metal electrode was due to less number of the ICs adsorbed on the surface. When the same LbL coating was applied to a copper substrate, the corrosion was suppressed even under an electrochemical cycling environment.
3. Results and Discussion
Figure 1 shows a schematic diagram of the LbL coating deposited on a metal surface. The LbL assembly can be formed because MMT is electrically negative and can be adsorbed on the electrically positive surface of the PEI-treated metal substrate via electrostatic attraction. This process was repeated to obtain the desired bilayer number by repeatedly immersing the sample in MMT and PEI solutions. As shown in the schematic diagram, PEI connects sheet-shaped MMTs, and small molecules in the solution would penetrate the coating by diffusion. When these molecules reach the metal surface, they will undergo oxidation or reduction reactions, depending on the voltage applied to the metal electrode. In this study, a redox couple (IC) was used, and its reversible redox reactions and corresponding molecular structure change with the transfer of two hydrogen and two electrons were shown in the schematic diagram.
Figure 2a shows the thickness of the LbL (MMT-PEI)
n (n = bilayer numbers) coating assembled using MMT dispersions of pH 4 and 10. The thickness increased as n increased: at n = 5, the thickness per bilayer (BL) was small (20 nm/BL), but, after that, the thickness per bilayer increased to more than 50 nm/BL. This growth rate is much higher than that of the previously reported LbL MMT-PEI coating (3 nm/BL) [
6], possibly because of the differences of the PEI molecular weight and the substrates used (i.e., poly(ethylene terephthalate) vs. gold) [
10]. It should be noted that the thickness of LbL (MMT-PEI)
n was hardly affected by the pH of the MMT dispersion solution.
However, the surface roughness was significantly influenced by the pH, and it can be seen that, at pH 10, the surface roughness increased more rapidly with the increase in the number of layers than at pH 4 (
Figure 2b). This result can be attributed to the pH-dependent structure of the cationic polyelectrolyte PEI (pKa = 6.5~8.5) [
7]. At high pH, it is in its coiled state. With the decrease in pH, it changes into a linear form due to the chain’s mutual repulsion. Therefore, the PEI layer already deposited at pH 10 on the electrode surface during the previous LbL process would spread out in plane when the electrode surface was exposed to the pH 4 MMT dispersion. However, it would remain aggregated if it were exposed to pH 10 MMT dispersion. Therefore, the surface roughness would be much higher for the LbL (MMT-PEI)
n deposited with pH 10 MMT solution. Unlike PEI, MMT’s sheet structure does not change significantly with pH, and the negative charge density of MMT is not notably affected by the change of pH from 3 to 11 [
11]. LbL (MMT-PEI)
n assembled with pH = 4 MMT solution resulted in a smoother surface, and it would be beneficial for blocking mass transport, serving as a barrier layer.
Figure 2c shows a typical surface of LbL (MMT-PEI)
n=10 coating assembled with pH 4 MMT solution. It resembles a cobbled surface with some thin wrinkles, possibly the thin layers of folded MMT. The cross-section (
Figure 2d) of the coating shows a wavy layered structure in which sheets are stacked to the thickness direction. This layered structure is similar to other clay-polymer thin films [
5]. As expected, a rough surface and cross-section were observed for LbL (MMT-PEI)
n=10 assembled with pH 10 MMT solution (see
Figure S1).
The electrochemical response of the LbL (MMT-PEI)
n coating in an IC solution was investigated using cyclic voltammetry. As shown in
Figure 3a, the CV of the uncoated gold electrode shows large and distinct oxidation and reduction peaks at 0.05 and 0.08 V (vs. Ag/AgCl), respectively, due to the redox reaction of IC on the electrode’s surface. Although the coated electrodes also show the faradaic peaks, the peak values are significantly smaller, demonstrating that fewer IC reach the surface. In addition, as the number of coating layers increases to 10 and 20, the redox peak value and the overall current decrease (
Figure 3b), indicating that the number of ICs reaching the surface is reduced with higher n. The corresponding blocking efficacy based on the reduction of the peak current reached up to 97.5%.
Compared to the bare electrode, it would take a longer time for IC in the solution to diffuse through the thickness direction of the LbL coating and to approach the metal surface. The reason is that MMT in the LbL (MMT-PEI)n is sheet shaped, and each layer is deposited in plane with the surface as shown in the SEM images. Furthermore, the coating also has a thickness of ~400 to 1200 nm and is made of a material with low conductivity. The potential difference between the oxidation and reduction peaks of IC in the coated electrode is within ~30 mV at a scan rate of 50 mV/s, which indicates that IC redox reaction is facile. Considering the slow diffusion of IC into the coating, this result suggests that IC adsorbed near the metal surface is responsible for the coated electrodes’ faradaic peaks.
CVs at different scan rates were measured (
Figure 3c) to determine whether IC is confined on the surface or dissolved in the solution. IC’s peak current values in the coated electrodes were linearly proportional to the scan rate with a slope of 1, indicating these peaks are from the adsorbed IC near the metal surface [
12]. In an uncoated gold electrode, IC’s peak current is proportional to the scan rate multiplied by 0.5, indicating that the IC redox reaction is diffusion controlled. Because the concentration of the surface-adsorbed species is linearly proportional to the peak currents, the reduced peak current values (to 0.45 from 6.06 μA/cm
2) indicate that the concentration of adsorbed species on n = 20 is significantly lower than that of n = 10.
Figure 3d shows that the peak current value gradually increases as the number of cycles of CV increases and reaches equilibrium value at about 50 cycles. This indicates that the amount of IC reacting on the surface gradually increases as the cycle is repeated. This result can be attributed to the slow diffusion of IC and water molecules into the coating and, possibly, the molecular rearrangement of the coatings under voltage application. LbL coatings made with MMT solution at pH 7 and 10 show the same trend. However, the peak currents are higher than that of the smoother coating made with pH 4 MMT solution. This means that the rougher surfaces of the LbL coatings made with higher pH facilitate the diffusion and adsorption of IC inside the coatings. In the coated sample, the shape of the CV remains unchanged after stabilization, and the LbL assembly is electrochemically stable like other LbL electrodes [
13,
14]. These results show that LbL coating is effective at blocking material transfer of IC, and its blocking degree can be controlled by the deposition conditions.
A copper electrode was coated with the LbL coatings made of a pH 4 MMT dispersion, and the corrosion caused by the reaction with chloride ions was investigated. The CVs of the bare copper and the coated copper electrodes in
Figure 4 show that an oxidation current and a reduction current occur, respectively, in the voltage range of −0.4 to 0.3 V. The reduction peak is between −0.08 to −0.06 V. This is due to the reaction of copper with chloride ions (Cu + Cl
− <-> CuCl + e
−) [
15,
16]. Bare copper has the largest reduction peak value and cathodic peak area. In the case of the coated electrode, it can be seen that the values decrease. When the number of layers is 15, the peak value is reduced to about a half, and when the number of layers is 30, it is reduced to about a third. This is close to the level that is reduced (1/2 ~ 1/10) when copper is coated with a common organic inhibitor for copper corrosion such as imidazole [
16]. This demonstrates that corrosion due to small ions such as Cl
− can be suppressed considerably with this type of LbL coating.