Figure 1 shows a cyclic voltammetry stripping analyzer (CVS) simulating the effect of the concentration of hydrolyzed collagen on the copper electrodeposition. From the Fig. 1(a), there is a increase of collagen concentration, while the amount of copper deposition is decreasing, and at the same time. when collagen is in the range of 0–10 mg/L, its slope decreases rapidly, and the change amplitude is large, the effect on the inhibition process of copper deposition is the most obvious, when the collagen concentration exceeds 10 mg/L, its inhibition effect is weakened. It indicates that collagen has an inhibitory effect on copper electrodeposition, and the inhibitory effect is enhanced with the increase of concentration, the optimal concentration is 0–10 mg/L.
The curve-fitting mathematical formula for CVS electricity consumption as a function of collagen concentration is given below, see Eq. (4).
$$y =18.7{{e}^{-}}^{\frac{x}{4.1}}+3.5 (4)$$
where \(x\) is the collagen concentration (mg/L); \(y\) is the amount of electrodeposition (mC)
As shown in Fig. 1(b), the first and second derivatives of Eq. (4) are analytically solved to find the optimal value of collagen in the acidic electrolyte. The results show that when the collagen is 2 mg/L, its inhibition effect begins to appear in the "consumption phase", such as the II. zone, when the collagen is 20 mg/L, its inhibition effect changes to the "smoothing phase", as shown in the III. zone.
This article simulates the degradation behavior of collagen in a static electrolyte, as shown in Fig. 2(b). In contrast, Fig. 2(a) shows the electrodeposition performance of a stable basal electrolyte, it can be seen from region A that the initial deposition potential is around 50 mV, and when 10 mg/L collagen is added, the initial deposition potential moves significantly negatively to around − 50 mV. With the extension of degradation time, the negative shift degree of the initial deposition potential decreases and changes towards the initial point of the basal electrolyte, simultaneously, the initial deposition potential of the polarization curve at 84 h and 128 h basically overlapped, indicating that with the increase of collagen degradation time, its inhibitory effect reaches saturation, and the initial deposition effect on copper is reduced. It can be seen from the slope of region B that the curve of 0 h addition of collagen is reduced compared with the slope of the curve of the basic electrolyte, and with the extension of time, the slope of the curve is further upgraded, and the polarization intensity is continuously weakened, indicating that the addition of collagen will enhance the inhibition of copper ion deposition; at the same time, the slope of the initial addition of collagen is the smallest, and the polarization intensity of the electrolyte is the highest, indicating that collagen will also undergo hydrolysis in the acid plating solution with the change of time. As shown in region C gives the copper electrodeposition stripping section curve, the oxidation peak of the basal electrolyte is near 220 mV, and with the extension of time, its oxidation peak gradually positively shifted to 280 mV, which indicates that the polarization strength of collagen is negatively correlated with the time of its own degradation.
Considering the accelerated collagen depletion during the electrodeposition process, the electrodeposition step is further added to the static loss, and plate electroplating is performed at 0 h, 2 h, 8 h, 16 h, 32 h, and 42 h, respectively. The degree of collagen consumption after each electrodeposition is monitored by CVS, as shown in Fig. 2(c), which is reflected as a decrease in potential. Based on the CVS data of this process, we find that the initial deposition potential of the polarization curves at 32 h and 42 h basically overlapped, the consumption phase is shortened by 1/2, indicating that the electrodeposition process is the decisive factor in the accelerated loss of collagen.
According to the formula (4) which the fitting relation of collagen concentration and electrodeposition amount, we obtain the relationship between the degradation time and collagen concentration of static electrolyte and static-electrodeposited electrolyte, respectively, corresponding to Figs. 3(b) and 3(d). The quantity of electrodeposition (Q) and collagen concentration obtained at different times using a static-electrodeposited state electrolyte are shown in Table 2. From Figs. 3(a) and (c), it can be seen that the amount of copper electrodeposition has a power exponential relationship with the degradation time of collagen, which further verifies the inhibitory effect of collagen on copper ion deposition. At the same time, when electrodeposition is carried out at 0 h, 2 h and 8 h, respectively, the degradation rate of collagen in the static-electrodeposited electrolyte is significantly higher than that in the static electrolyte. After 8 h, that is, when the collagen concentration is lower than 2.41 mg/L, the gap between the two narrows and the efficacy is similar, which indicates that the electrodeposition process will increase the loss of collagen and make its effect worse. Therefore, monitoring the collagen concentration during electrodeposition is extremely important to maintain the stability of copper foil in industrial processes. The practical significance of formula (4) is that we can calculate the existing collagen concentration by detecting the amount of electrodeposition and use the formula to maintain it at the optimal concentration, thereby reducing the rate of waste foil and improving economic benefits.
Table 2
The relationship between the degradation time of static-electrodeposited electrolyte and the amount of copper electrodeposition/collagen concentration
Time(h)
|
Non-additive
|
0
|
2
|
8
|
16
|
32
|
42
|
Q(mC)
|
25.08
|
6.72
|
8.91
|
15.52
|
18.97
|
24.62
|
24.75
|
Collagen concentration(mg/L)
|
0
|
7.79
|
5.68
|
2.41
|
1.37
|
0.09
|
0.06
|
The mechanism of collagen hydrolysis is shown in Fig. 4. Collagen is a macromolecular amino acid polymers, which will be decomposed into a variety of amino acids in the acid plating solution, the amino group is converted to positively charged NH3+, selectively seizes the active point on the surface of the cathode plate to inhibit the reduction and deposition of copper ions, at the same time, in the deposition process, collagen will continue to adsorb on the surface of the copper foil protrusion, future inhibit the growth of copper grains, thereby playing a role in refining the grains and making the coating more flat. However, due to the competitive adsorption of Cu2+, the charged groups produced by collagen are desorbed, and the efficacy gradually disappears.
In addition, by laser confocal Raman spectroscopy to analyze the chemical composition of the copper foil surface, the gap in Raman peak position revealed the presence of collagen, as shown in Fig. 5. The strongest Raman peak with copper oxide at 235 cm− 1 and a weaker peak near 624 cm− 1 corresponded to the telescopic vibration mode of the Cu-O bond, and a Raman peak with copper oxide at 1090 cm− 1 corresponded to the change of O-Cu-O bond angle [17]. At the same time, we found the amide I peak at 1583 cm− 1, corresponding to C = O telescopic vibration (~80%) and N-H bending vibration (~20%), which indicates that hydrogen bonds are formed between N-H stretching and C = O. The amide II band is generated by the coupling of N-H bending vibration and C-N tensile vibration [18, 19]. The highest peak of amide III band appears in 1250 cm− 1, which is mainly related to C-N telescopic vibration and N-H bending vibration (about 30% each), C-C stretching (~20%) and C-H bending (~10%) [20–23], and the existence of three amide bands prove that the surface of the prepared copper foil adsorbs organic matter, which confirm the successful introduction of collagen.
The micromorphology of copper foil in the process of collagen deposition is shown in Fig. 6, it can be seen that Fig. 6(a) shows the surface morphology when collagen is not added to the electrolyte, its surface is relatively rough, a large number of bulk crystal particles appear, showing a clear peak-valley structure [15]. Compared with Fig. 6(a), when 10 mg/L collagen is added to the electrolyte, as shown in Fig. 6(b)-(d), the surface of the copper foil becomes flat, the bulk crystal particles are reduced, and many fine nodular-like crystal particles appear, When preparing copper foil at 8 h, the nodular particles are the densest, the surface is the flattest, and the roughness is reduced, which is because the addition of collagen can inhibit copper deposition, and will play a role in refining the grains during the electrodeposition process; with the progress of electrodeposition, collagen is continuously consumed, and the rate of diffusion into the valley on the cathode surface is less than the rate of reaching the micropeak, so that the concentration of collagen on the valley is lower, and the copper deposition is faster here, thereby achieving the effect of flatting [24]. As shown in Fig. 6(e)-(f), the bulk crystal particles on the surface of the copper foil gradually get bigger, forming the same micromorphology as the surface of the copper foil which prepared by basal electrolyte, indicates that when the collagen content is lower than 2.41 mg/L, its refinement effect on the grains is weakened, which in turn leads to a decrease in tensile strength.
XRD analysis of ECF obtained by electrodeposition during collagen degradation is shown in Fig. 7. We can see three diffraction peaks appear, compared with the Cu standard card, they are (111), (200), (220) crystal planes. When collagen is first added, the intensity of the diffraction peak is enhanced, after a period of collagen degradation, the diffraction peak intensity begins to decrease, and the concentration is 2.41 mg/L (t = 8 h) to the minimum; with the further degradation of collagen, the effect of collagen gradually disappears, the additive effect deteriorates, and the intensity of the diffraction peak increases. When the action time of collagen reaches 8 h, the intensity of the diffraction peaks of the crystal plane of (111), (200) and (220) is maintained at the minimum peak, indicating that the adsorption of collagen on the cathode surface reaches saturation, the surface of copper foil is flat, and the fine grain strengthening effect is optimal.
In order to accurately analyze the variation of different texture optimization degree with degradation time under the collagen participation reaction, the texture coefficient of each crystal plane is calculated by Scherrer's formula, as shown in Fig. 8, when electrodeposition is carried out at different times, the proportion of copper foil (111) crystal planes is higher than that of (200) and (220) crystal planes. When no additives are added to the system, the crystal planes of (111), (220) and (200) account for 39.36%, 30.11% and 30.53%, respectively; when collagen is just added, the proportion of (220) crystal plane decreases, and the proportion of (200) crystal plane increases, which indicates that the addition of collagen leads to the transformation of the preference orientation of crystal plane from (220) to (200). At the same time, during the degradation process, collagen is initially promoted to grow copper foil along the crystal plane of (111) and (200) to inhibition, but the proportion of (200) crystal plane continues to increase after 8 h. With the degradation process, (220) crystal plane growth shifts from inhibition at the beginning to promotion, and turns into inhibition when the collagen content is less than 2.41 mg/L, which means that the effect is ineffective after the collagen content less than 2.41 mg/L, and the basal electrolyte begins to exert its role.
Figure 9 shows the transmission electron microscopy (TEM) images and diffraction patterns of copper foils in the presence and absence of collagen. Firstly, the microstructure of the foil before and after the addition of collagen is compared, and it is found the grain size reduced significantly, indicating that the addition of collagen can refine the grains. The diffraction patterns of the copper foil before and after the addition of collagen show the characteristic (111) crystal plane of FCC copper, which is in agreement with the XRD of Cu as shown in Fig. 9(a) and (c). From Fig. 9(b), we can see the obvious twinning organization, that is because Cu belongs to the FCC metal with low intrinsic layer error energy, which easy to promote the growth of twinning, and a large number of twin boundaries(TBs) appeared in the grains, and TEM shows that a large number of TBs exist in the grains, and most of the grains are further subdivided into a twinning/matrix laminar structure through the high-density twinning boundaries[25], meanwhile, the twinning space is narrowed down from 96 nm to 20 nm after the addition of collagen, as shown in Fig. 9(d), which is due to the fine grain strengthening effect greatly improves the density of the twinning boundaries and forms a staggered high-density dislocation zone, which further hinders the plastic deformation of the copper foils and improves their tensile strength [26–28], a finding that skillfully combines the organizational effects of collagen on copper foils with the aspects of mechanical properties. Therefore, the addition of collagen can greatly reduce the grain size and change the preferential orientation of the grains, at the same time, promote the development of twinning boundaries, thus improving the tensile strength of the copper foil as well as the electrical conductivity and other physical properties.
The roughness of ECF is characterized by Rz as shown in Fig. 10, it shows a first decrease and then an increase. The roughness of ECF prepared without adding collagen is 3.79 ± 0.29 µm, and the roughness is reduced to 2.99 ± 0.25 µm when collagen is added at 0 h. This is attributed to the inhibitory effect of collagen, which suppresses the surface bulk particles and grows some small size nodular particles, lead to the reducing the size of the surface grains, and playing an effect of refining the grain size, thus reducing the copper foil surface roughness. With the increase of degradation time to 8 h, the roughness of copper foil prepared by electrodeposition is reduced to 2.71 ± 0.07 µm, which is due to the inhibition of bulk particles in the electrodeposition process, replaced by the growth of small-size nodular particles, and the crystallization is more dense, resulting in a substantial increase in the surface flatness of copper foil [29]. When the time exceeds 8 h, the collagen content is lower than 2.41 mg/L, the roughness increases again, which is due to the deterioration of the collagen efficency, adsorption to raised areas of the surface gradually invalidates, and the growth of the large-size grains aggregated on the (111) surface again, the decrease of the leveling property leads to the increase of the roughness, which corresponds to the results of the SEM [27]. However, the roughness could not reach the initial value due to the failed resdual collagen and adhered to the surface of the copper foil, resulting the number of bulk particles growing does not reach the number of electrolytic copper foil without added collagen.
The tensile curve is shown in Fig. 11(a), the corresponding stress-strain curve of ECF which prepared under 2.41 mg/L collagen can be clearly observed, and the tensile strength and elongation are in the highest state, indicating that the copper foil prepared by electrodeposition at 8 h has high mechanical properties. It can be seen from Fig. 11(b) that with the extension of time, the tensile strength and elongation first increase and then decrease, this is consistent with the opposite trend of roughness change and mechanical property change mentioned by Yi et al [2]. The tensile strength of the ECF prepared without collagen addition is 263.9 MPa and the elongation is 3.04%, when the collagen content is 2.41 mg/L, the tensile strength of the copper foil increases to 317.0 MPa and the elongation increases to 3.33%, combining with roughness and microscopic morphology, we find that collagen reduces the surface roughness due to its inhibitory effect at 8 h, makes the grain smaller, fine grain strengthening enhances tensile strength and elongation at the same time, with the gradual extension of reaction time, tensile strength and elongation will continue to decrease, the performance of samples prepared at 42 h and the initial value is basically the same, which indicates that the overall action time of collagen in the process of electrodeposition is 42 h.
In order to investigate whether the degradation of collagen affects the conductivity of the prepared copper foil, conductivity test is performed at room temperature to illustrate this problem, as shown in Fig. 12. In general, twinning boundaries are interfaces with high electron migration and are conducive to conduction[30]. The conductivity is affected by copper foil particles in a similar way to the mechanical properties, and it can be proved that collagen changes the growth of copper foil.