Characterization and Electrochemical Investigation of Heterogeneous Sb-Cu Coatings
Round 1
Reviewer 1 Report
Report on the manuscript coatings-2572057-peer-review-v1 entitled “Characterization and Electrochemical Investigation of Heterogeneous Sb-Cu Coatings”.
The submitted manuscript should be revised. In this work, the electrodeposition of antimony-copper (18-30 %) alloy and checked through SEM, AFM and TEM as well as phase structure, roughness and hardness. The electrochemical properties were investigated by impedance and polarization measurements in acidic solution. In short, the following points should be addressed:
1. The language of the manuscript should be revised. In addition, scientific expression such as “impedance electrochemical spectroscopy” should be “electrochemical impedance spectroscopy”.
2. The abstract should focus on what was reported in the submitted manuscript and the first statement is away from this.
3. In keywords: “heterogeneity;” should be “heterogeneous alloy.”
4. In the discussion of Fig. 2B, the reason of cracking is written as “It is possible that they are the result of strong internal stresses in the layers due to the significant difference in atomic radius of two metals”. This should have reference as there are many reports with different metals and haven’t like this morphology.
5. In line 211, there is editing errors before 2θ. In addition, in line 212, there is another editing errors before (122) plane which should be “and”.
6. At the end of XRD discussion, “The results show 73 % Sb and 27% Cu2Sb in SbCu18 layer”, the authors should indicate how they estimate this percentage supported by references.
7. The XRD JCPDS cards of Sb and Cu2Sb should be supported with optimum references.
8. In EIS, the authors should indicate why they selected this equivalent circuit and define its parameters.
9. Conclusion should be clearer, and it should have clearly what achieved in this work.
It should be revised.
Author Response
Response to Reviewer 1 Comments
Thank you very much for the thorough and careful review. We appreciate your time spent for reviewing our submission. The comments and remarks are quite appropriate and comprehensive. They will contribute to the quality of our paper and also for our future research.
Point 1: The language of the manuscript should be revised. In addition, scientific expression such as “impedance electrochemical spectroscopy” should be “electrochemical impedance spectroscopy”.
Response 1: We have carefully revised the English. The error pointed out by the reviewer has been fixed.
Point 2: The abstract should focus on what was reported in the submitted manuscript and the first statement is away from this.
Response 2: We thank the reviewer for his opinion. We totally agree with him. However, we have complied with the cover template, according to which the authors are strongly encouraged to use the following style of structured abstracts, but without headings: (1) Background: Place the question addressed in a broad context and highlight the purpose of the study;…’.
Point 3: In keywords: “heterogeneity;” should be “heterogeneous alloy.”
Response 3: We thank the reviewer for the suggestion. We have changed the mentioned keyword.
Point 4: In the discussion of Fig. 2B, the reason of cracking is written as “It is possible that they are the result of strong internal stresses in the layers due to the significant difference in atomic radius of two metals”. This should have reference as there are many reports with different metals and haven’t like this morphology.
Response 4: Many thanks to the reviewer for pointing out the incorrect statement. After a more thorough literature survey on crack initiation mechanisms, we revised our statement regarding the presence of strong internal stresses in the coatings. In the revised version of the manuscript, we relate the cracks to the high overpotential required for electrodeposition of high-antimony alloy coatings. These statements are based on the theory of island growth in electrodeposition and are supported by relevant references.
Point 5: . In line 211, there is editing errors before 2θ. In addition, in line 212, there is another editing errors before (122) plane which should be “and”.
Response 5: We are very grateful to the reviewer for his careful reading of our work. In the revised version, the errors have been fixed.
Point 6: At the end of XRD discussion, “The results show 73 % Sb and 27% Cu2Sb in SbCu18 layer”, the authors should indicate how they estimate this percentage supported by references.
Response 6: Thanks to the reviewer for the correct question. The quoted results indicate a higher content of the antimony phase, and hence of the total content of the antimony element, in comparison with the results of the X-ray fluorescence analysis. This shows an increase of the Sb content with increasing layer thickness. In the revised version of the manuscript, we have significantly expanded the discussion of XRD results in terms of crystallinity and compared them with metallurgically obtained alloys, including new references.
Point 7: The XRD JCPDS cards of Sb and Cu2Sb should be supported with optimum references
Response 7: JCPDS cards are included, additional references for XRD of Sb and Cu2Sb are given.
Point 8: Conclusion should be clearer, and it should have clearly what achieved in this work.
Response 8: We have made efforts to improve the conclusion by rewording some phrases.
Reviewer 2 Report
Comments on the manuscript (coatings-2572057) with the title "Characterization and Electrochemical Investigation of Heterogeneous Sb-Cu Coatings", by Vasil Kostov and Boriana Tzaneva.
The authors describe the electrochemical deposition of copper antimony layers. By varying the current density, they can influence the Cu content, whereby the copper should always be present in the Cu2Sb phase. They have characterized their layers with SEM, XRF, EDX and AFM. Furthermore, they have determined the microhardness and studied the electrochemical behaviour by impedance spectroscopy. In the introduction, the authors have already pointed out the possible wide range of applications of Cu-Sb coatings. The manuscript is very well suited for the journal Coatings. However, in my opinion, changes or clarifications are needed before publication.
Before I get to my questions and comments, I would like to say that I have not evaluated the impedance results, as I am not an expert in this field.
In the experimental part, the authors write that they used platinum or brass as a substrate for the deposition. In the corresponding results, they only show the results for platinum. Are the deposition characteristics on brass comparable to those on platinum?
In their model, the authors describe that during deposition, copper is deposited first and this reduces the overpotential. Could the authors detect the copper layer, e.g. with EDX or XRF, if the substrate is removed from the electrolyte after this time period?
The authors nicely show that the current density at the cathode has an influence on the composition of the layers and the copper content becomes less at higher densities. Do the authors have any idea why this effect appears?
The authors used XRF to determine the weight percentages of copper in the layers. Comparing the XRF results for the SbCu18 film with the EDX point analysis (Figure 3e), the results do not match at first sight. Is the reason only the mentioned inhomogeneous distribution of the elements? What do e.g. further point analyses at other locations show? Furthermore, for SbCu18 the special layer structure (Figure 3f) was pointed out. The BSE image shows by different brightness that the layers must have different compositions. Is it now possible that the bright layers could also be almost pure antimony layers? Furthermore, the XRD results could perhaps be used to determine the weight percentages for comparison.
In discussing the XRD results, the authors give the ratio between Sb and Cu2Sb only for the SbCu18 film. What are the ratios for the other films? In addition, Scherrer's equation could be used to determine the size of the crystallites in the layers. This would provide additional information. For me, the microhardness results indicate that the crystal size should not change much.
With the help of laser ablation the authors were also able to generate nanoparticles, but here I lack information about the composition of the particles. In one sentence the authors write about antimony particles and in another sentence about Sb-Cu particles. Since here from an inhomogeneous surface the particles were produced, one cannot assume in my opinion that all particles have the same composition. Were diffraction experiments also made to the particles in the TEM or afterwards with the XRD?
In section 3.4, the authors explain that with anodic polarization at 1 V, the surface is more corroded in the antimony-rich regions. Here, in my opinion, an EDX point analysis would also be useful to quantify this effect, since this is difficult to see from the EDX maps. This would also help to see that the Cu2Sb phase is more stable. I wonder how reference 20 helps in this case, since this paper talks about copper dissolution and not about antimony dissolution.
Another point that struck me is that this is the first time it has been said that the concave regions are dominated by the Cu2Sb phase. This should perhaps already be introduced in section 3.2.
Finally, I have two minor comments
I am missing a motivation why the electrolyte with C4H4O6 is used. Is this due to the antimony precursor used? What advantages does this electrolyte have over other common electrolytes?
In Figure 2, you can't read the numbers. In the caption, the corresponding side lengths were given but no information about the maximum height was given. Furthermore, CuSb82 should be changed to SbCu18.
Author Response
Response to Reviewer 2 Comments
We thank the reviewer for his helpful comments. We have tried our best to improve the manuscript with the changes described.
Point 1: In the experimental part, the authors write that they used platinum or brass as a substrate for the deposition. In the corresponding results, they only show the results for platinum. Are the deposition characteristics on brass comparable to those on platinum?
Response 1: We thank the reviewer for pointing out the omission regarding the used substrates. In the preliminary galvanostatic tests we used a platinum electrode. Changing the substrate from platinum to brass did not change either the potential-time dependences or the structure of the coating. For this reason, for all subsequent tests we used the brass as the substrate. This clarification has been added in the revised version at the end of the section 3.1. Electrodeposition of Sb-Cu Layers.
Point 2: In their model, the authors describe that during deposition, copper is deposited first and this reduces the overpotential. Could the authors detect the copper layer, e.g. with EDX or XRF, if the substrate is removed from the electrolyte after this time period?
Response 2: We thank to the reviewer for the interesting question. Indeed, we have deposited a thick Sb-Cu coating on a substrate without pretreatment, which allowed us to peel it off. At the substrate-coating interface the color of the coating is a pinkish and similar to a pure copper coating. Therefore, we have examined the substrate-coating interface by EDX of cross-section. These results are added in revised version and show increased copper content at the substrate-coating interface. Moreover, we have extended the discussion to the results of EDX and XRD top surface analyses, which also strongly indicate a lower copper content in outer layer than the average.
Point 3: The authors nicely show that the current density at the cathode has an influence on the composition of the layers and the copper content becomes less at higher densities. Do the authors have any idea why this effect appears?
Response 3: In electrodeposition of alloys, the content of the less noble metal usually increases with the increase of the current density or of the overvoltage [https://doi.org/10.1007/978-1-4419-9669-5_10]. In the particular case of Sb-Cu alloy coatings, the copper is the nobler metal and would preferentially deposit at higher Cu2+ concentrations in the electrolyte and/or lower cathodic current density (weaker cathodic polarization).
Point 4: The authors used XRF to determine the weight percentages of copper in the layers. Comparing the XRF results for the SbCu18 film with the EDX point analysis (Figure 3e), the results do not match at first sight. Is the reason only the mentioned inhomogeneous distribution of the elements? What do e.g. further point analyses at other locations show? Furthermore, for SbCu18 the special layer structure (Figure 3f) was pointed out. The BSE image shows by different brightness that the layers must have different compositions. Is it now possible that the bright layers could also be almost pure antimony layers? Furthermore, the XRD results could perhaps be used to determine the weight percentages for comparison.
Response 4: Many thanks to the reviewer for pointing out the incomplete explanation of our results. The XRF results were obtained at several points on the surface and reflect the content of the elements in a relatively large volume (area of 100 μm and the entire thickness of the layer). At the same time, the EDX point analysis gives information of a volume of about 1 μm3. Indeed, comparing the results of the two methods shows a different composition in the coating depth - at the substrate-coating interface the copper content is higher, while in the outer layers it is below the average (Table 1). The cross-section was observed in the SE mode, since in the BSE mode the layered structure was not so clearly observed. Additionally, EDX of cross-section shows that the lighter regions are richer in copper. In the revised version of manuscript, we have added EDX line analisis of cross-section of the coating as well as expanded the discussion of differences in elemental content from XRF, EDX, and XRD analyses.
Point 5: In discussing the XRD results, the authors give the ratio between Sb and Cu2Sb only for the SbCu18 film. What are the ratios for the other films? In addition, Scherrer's equation could be used to determine the size of the crystallites in the layers. This would provide additional information. For me, the microhardness results indicate that the crystal size should not change much.
Response 5: We thank the reviewer for the suggestion to determine the crystallite size using the Scherrer’s equation. The results showed a gradual increase in crystallite size and we could not relate it to the microhardness, which does not change significantly. The ratio of Sb-phase to Cu2Sb for all layers shows an increased amount of antimony compared to the average composition of the layers. Therefore, we have given a detailed example only of the most pronounced deviation. This greatly enriched our work. In the revised version, this information has been added as well as new Table 2 with a crystallite size of Sb- and Cu2Sb-phase.
Point 6: With the help of laser ablation the authors were also able to generate nanoparticles, but here I lack information about the composition of the particles. In one sentence the authors write about antimony particles and in another sentence about Sb-Cu particles. Since here from an inhomogeneous surface the particles were produced, one cannot assume in my opinion that all particles have the same composition. Were diffraction experiments also made to the particles in the TEM or afterwards with the XRD?
Response 6: We thank to the reviewer for the correct remark. Indeed, the TEM results show agglomerates of mixed type Sb and Cu2Sb (Figure 5a). The antimony particles are dominant, one of which is represented in Figure 5b. Some corrections in this regard have been made in the revised version.
Point 7: In section 3.4, the authors explain that with anodic polarization at 1 V, the surface is more corroded in the antimony-rich regions. Here, in my opinion, an EDX point analysis would also be useful to quantify this effect, since this is difficult to see from the EDX maps. This would also help to see that the Cu2Sb phase is more stable. I wonder how reference 20 helps in this case, since this paper talks about copper dissolution and not about antimony dissolution.
Response 7: Our conclusion about the stronger attack of the antimony phase is made mainly on the SEM observations. According to them, the concave regions proved to be a Cu2Sb phase remain almost unchanged compared to the surface before anodic polarization (Figure 9c). At the same time, the antimony crystals are completely obliterated, and these areas have acquired a well-defined porous structure. EDX maps rather shows that the more attacked areas (convex) have increased antimony content. Since the attack is local (in the form of pores), the point analysis does not give serious deviations in the content of the elements in the convex and concave areas. In the attacked antimony phase, the antimony content remains around and above 90%, while the copper content in the indented areas is around 15%. This clarification has been added in the revised version.
Reference [20] seems important because it is the only study in which the anodic behavior in acidic media of a Cu-5% Sb alloy and pits formation is discussed. Since in the studied alloy in [20] the antimony is apparently in the form of a bimetallic phase with the copper and shows increased resistance. Hance, we concluded that the bimetallic CuxSby compounds have a higher resistance to anodic polarization than the pure metals Cu and Sb.
Point 8: Another point that struck me is that this is the first time it has been said that the concave regions are dominated by the Cu2Sb phase. This should perhaps already be introduced in section 3.2.
Response 8: We thank the reviewer for pointing out the insufficient consideration of the inhomogeneous distribution of elements on the coating surface. Attention to the inhomogeneous distribution of copper is reflected in section 3.2 in the discussion of EDX point analysis. Now the discussion on the non-uniform distribution of copper both on the surface and in the depth of the coatings is expanded. Additionally, we discussed together the results of EDX and XRD analyzes and indicated that the convex crystalline zones are of pure antimony phase, while the Cu2Sb phase is concentrated in the smooth depressions.
Point 9: I am missing a motivation why the electrolyte with C4H4O6 is used. Is this due to the antimony precursor used? What advantages does this electrolyte have over other common electrolytes?
Response 9: We thank to the reviewer for the important question. Tartaric acid C4H6O6 is an additive to increase the solubility of the antimony salt. Additionally, we use methanesulfonic acid as an alternative to sulfuric acid. In addition to high conductivity, this acid is biodegradable and safer to use [https://doi.org/10.1039/A900157C]. Moreover, the overvoltage of copper deposition in methanesulfonate electrolyte is significantly lower, and copper coatings have finer crystallites compared to coatings deposited from sulfate electrolyte [https://doi.org/10.15421/082001]. However, the most significant advantage of the developed electrolyte is the possibility of electrodepositing alloy coatings with a high antimony content. This information has been added in the revised version of the manuscript.
Point 10: In Figure 2, you can't read the numbers. In the caption, the corresponding side lengths were given but no information about the maximum height was given. Furthermore, CuSb82 should be changed to SbCu18.
Response 10: The corrections have been made.
Reviewer 3 Report
The manuscript presents detailed study of phase structure, roughness, hardness, and electrochemical properties. It demonstrates potential applications for catalytic layers or binder-free electrode. Minor revision is suggested. There are a few suggestions that the author may consider:
- The authors claim that cracks in Figure 3 is the result of strong internal stress (Line 198). It is suggested to provide evidence to support the claim. Further, please provide possible explanation why the crack is not obvious in CuSb18.
- Please provide theoretical support why copper only exists in Cu2Sb phase.
- While the authors shows TEM images in Figure 5, it is necessary to label the inter-planar spacing in Figure 5b to determine the nanoparticle structure.
- To study the structural change during anodic polarization, It is suggested to provide XRD comparison before and after electrochemical test.
Minor problems:
- It seems there is a crack in SbCu30 (Figure 3a). Please provide a low magnification image for SbCu30 for fair comparison of the plan-view morphology.
- In Figure 1(a), CuSb72 should be CuSb74. Please check
Author Response
Response to Reviewer 3 Comments
We appreciate the careful work of the reviewer and hope that the corrections will be met with approval.
Point 1: The authors claim that cracks in Figure 3 is the result of strong internal stress (Line 198). It is suggested to provide evidence to support the claim. Further, please provide possible explanation why the crack is not obvious in CuSb18.
Response 1: We thank to the reviewer for noticing the omission. In the revised version, we have expanded the discussion of the origin of internal stresses and added two references.
Surface cracks are present in all coatings. In Figure 3d, there are two cracks in the SbCu18 (one starts at the top right corner), but they are more difficult to notice. However, we hope that at the higher magnification (Figure 3e), the crack is clearly visible in the lower right corner.
Point 2: Please provide theoretical support why copper only exists in Cu2Sb phase.
Response 2: In the revised version, we have extended the discussion to the alloys recorded in the coatings and explained the composition through the phase diagram of a copper-antimony binary alloy [https://doi.org/10.1007/s00706-012-0737-1]. According to it, at an antimony content above 33 at.% (or 48.5 wt.%) the alloy contains two phases - a pure antimony phase and copper in the form of Cu2Sb.
Point 3: While the authors shows TEM images in Figure 5, it is necessary to label the inter-planar spacing in Figure 5b to determine the nanoparticle structure
Response 3: Missing information was added as recommended by the reviewer.
Point 4: To study the structural change during anodic polarization, It is suggested to provide XRD comparison before and after electrochemical test
Response 4: We thank to the reviewer for the suggestion. We expect no change in the two main phases of coatings (Sb and Cu2Sb). During anodic polarization an antimony oxides are formed, which could possibly be registred by XRD. Unfortunately, we are not able to conduct further analyzes to confirm this hypothesis.
Point 5: It seems there is a crack in SbCu30 (Figure 3a). Please provide a low magnification image for SbCu30 for fair comparison of the plan-view morphology.
Response 5: Figure 3a has been replaced with a new one at a lower magnification.
Point 6: In Figure 1(a), CuSb72 should be CuSb74. Please check.
Response 6: Many thanks to the reviewer for pointing out the error. The labels in Figure 1a have been replaced with the correct ones.
Round 2
Reviewer 1 Report
Accepted
Reviewer 2 Report
Comments on the revised manuscript (coatings-2572057) with the title "Characterization and Electrochemical Investigation of Heterogeneous Sb-Cu Coatings", by Vasil Kostov and Boriana Tzaneva.
First, I would like to thank the authors for their detailed answers to my comments and questions. The authors have revised this manuscript and used my questions and comments to make improvements. In my opinion, the manuscript can now be published in the journal Coatings. I cannot say whether a revision of the English language is still necessary.
