Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript reports the effects of Er and Al co-doping on the photocatalytic efficiency of sol-gel derived ZnO nanoparticles for MO removal. It concludes that the co-doping tactic results in an enhanced visible light illumination and increased BET specific surface area, which contributes to the photocatalytic activity.

The manuscript is clearly presented, and the experiment design is in general reasonable. It can be recommended publication after a major revision considering the follows.

1. The photocatalytic experiment did not distinguish clearly the dark adsorption from the photocatalytic degradation. The photocatalytic degradation should begin after establishing the dark adsorption equilibrium, after which, a reaction rate constant, probably following a pseudo first order kinetics, could be achieved and compared for each specimen. Please refer to DOI: 10.1016/j.apsusc.2020.146779 for details.

2. The low temperature nitrogen adsorption-desorption isotherms for each specimen to evaluate the BET specific surface area should be provided.

3. Also, please include the UV-Vis DRS in Figure 3.

4. The cycling performance should be provided, as photo-corrosion is a major challenge for ZnO photocatalysis.

5. Figure 1 provides less information and could be deleted.

6. Existing literatures on Er/Al co-doping ZnO, although in thin films, should be mentioned clearly in the Introduction.

Author Response

We sincerely thank the reviewer for the careful reading of our manuscript and for the valuable comments, which helped us to improve the quality and clarity of the work. Below, we provide a point-by-point response.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This study reported the synthesis of ZnO and ZnO nanoparticles doped with 1.5 at.% Er, 5 25 at.% Al, and 1.5 at.% Er:5 at.% Al using the sol-gel methodThere are some comments for authors improving the quality of manuscript.

1. It is non-standard for the author of Figure 3 to number the different XRD curves together with other characterization results.
2. The kinetic curves of photocatalytic performance should be provided.
3. The author's explanation of the mechanism behind the enhanced photocatalytic performance is still quite insufficient. 
4. Furthermore, in the introduction, the author should specifically emphasize the purpose and function of modification.
5. In addition, the author should compare the performance of the material with that reported in other literatures.

Author Response

We sincerely thank the reviewer for the careful reading of our manuscript and for the valuable comments, which helped us to improve the quality and clarity of the work. Below, we provide a point-by-point response.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Comment 1: How does Erbium doping narrow the bandgap of ZnO? What is the rationale for co-doping with both Er and Al? The band energy diagrams for all samples should be illustrated and compared with literature references alongside the experimental data presented in the manuscript.

Comment 2: In the synthesis section, how was the annealing temperature of 400 °C determined to be the optimal pre-treatment temperature for the cured samples after sol–gel synthesis? Were other annealing temperatures tested? In addition, the annealing atmosphere and temperature ramp rate should be specified in the experimental section.

Comment 3: How were the bacteria separated from the nanoparticles after the photocatalytic treatment? This step could greatly influence the results of the antibacterial testing of the ZnO nanoparticle samples.

Comment 4: A statistical analysis (e.g., t-test or ANOVA) should be performed on the antibacterial data (Figure 5) to evaluate whether the differences between the ZnO nanoparticle samples are statistically significant.

Comment 5: The figure and plot quality and spacing should be improved for the readers.

Author Response

We sincerely thank the reviewer for the careful reading of our manuscript and for the valuable comments, which helped us to improve the quality and clarity of the work. Below, we provide a point-by-point response.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript improves; but further revisions are still required.

1. Figure 3a, what’s the cause of the two minor peaks in the range 500-700 nm? Figure 3b, for ZnO-Er, the blue line, the band gap should be a little larger than indicated. Also, please label the band gap for all samples in the figure.
2. The Introduction section is too lengthy, which could be shortened to focus on the doped ZnO for photocatalytic applications.
3. I cannot find the specific surface area data in the manuscript.
4. Figure 4a, the unit for the x-axis is incorrect (“h” should read “min”); Figure 4b, the first point for all specimens should be zero, because for the fitting, the C₀ point should be set at the concentration just after the dark adsorption equilibrium.

Author Response

We would like to thank the reviewer for the careful evaluation of our manuscript. Below, we address each of the comments raised, providing clarifications, corrections, and updates to the manuscript where necessary

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for the revised manuscript and responses. My previous comments have been mostly addressed. However, please include the ANOVA result tables into the supplemental document. I recommend the manuscript can be accepted for the publication after this minor changes. 

Author Response

We sincerely thank you for your valuable comments and for recommending our manuscript for publication. As suggested, we have included the complete ANOVA results tables in the supplementary material.