In Situ Growth and UV Photocatalytic Effect of ZnO Nanostructures on a Zn Plate Immersed in Methylene Blue

Round 1

Reviewer 1 Report

In this manuscript, “In situ growth and UV photocatalytic effect of ZnO nanostruc- 2 tures on a Zn plate immersed in methylene blue”. The authors have performed the characterization study and the application part is acceptable manner. However, there are some minor issues that need to be rectified before the acceptance. Abstract: In the abstract, the results of the research must be briefly described.

1.         In Fig.2, Elemental mapping, the atoms should be mentioned in the figure.

2.         Authors should include survey spectrum of XPS analysis.

3.         Authors should calculate band gap energy value of the prepared photocatalysts.

4.         The authors should clearly explain the innovation, research gap, market gap, market demand, and importance of their work in the manuscript's introduction. They should justify the value of the work and compare their work with previously similar published papers. They should develop the electrocatalysis advantage and applications compared to other known systems. The introduction section needs to be elaborated.

10.1016/j.chemosphere.2022.136123, https://doi.org/10.1039/D1NJ03410C https://doi.org/10.1149/2754-2726/ac5c7a

 

 

Author Response

In this manuscript, “In situ growth and UV photocatalytic effect of ZnO nanostructures on a Zn plate immersed in methylene blue”. The authors have performed the characterization study and the application part is acceptable manner. However, there are some minor issues that need to be rectified before the acceptance. Abstract: In the abstract, the results of the research must be briefly described.

1. In Fig.2, Elemental mapping, the atoms should be mentioned in the figure.

We thank the reviewer for pointing this out. We have added the name of the atoms in the figure.

2. Authors should include survey spectrum of XPS analysis.

We have added the XPS survey spectrum in Figure 4(a).

3. Authors should calculate band gap energy value of the prepared photocatalysts.

We have estimated the bandgap of the ZnO nanostructures using Tauc plot and added it to the results and discussion section of the manuscript.

4. The authors should clearly explain the innovation, research gap, market gap, market demand,and importance of their work in the manuscript's introduction. They should justify the value of the work and compare their work with previously similar published papers. They should develop the electrocatalysis advantage and applications compared to other known systems. The introduction section needs to be elaborated.

We thank the reviewer for this suggestion. We have added the following sentences to the introduction to highlight the importance of our work

“In recent years, there has been much interest in environmentally benign nanostructure synthesis method. Especially in nano-photocatalysis, where the intended application is environmental remediation, it is desired that their synthesis approaches do not involve the use of toxic chemicals. Also, ZnO nano-photocatalyst synthesis methods need to be cost-effective to promote their wide-spread use in water treatment. This means the synthesis methods should be simple and not require expensive equipment.” 

“In other words, we designed a single-step approach where the photocatalyst synthesis and its application happen simultaneously.”

The main advantages of this nano-photocatalyst synthesis method are that it does not require high temperatures or any chemical additives, and it is simple, low-cost, and environmentally friendly. Moreover, the ZnO nanostructures in our photocatalyst design are attached to the Zn surface. Researchers have given much emphasis to the immobilization of photocatalysts to avoid agglomeration of the nanostructures, secondary pollution, and nanostructure reclamation from the water, which could be expensive. Our method achieves photocatalyst immobilization in a single step."

Reviewer 2 Report

Zn plates were immersed in an aqueous solution of MB, and an in situ ZnO nanostructure growth and UV photocatalytic effect were investigated. One of the significant advantages of this method is that it does not require any chemical additives, the developed materials were characterized by a wide spectrum of various techniques（SEM、EDS、XRD、XPS）which allowed to recognize their properties. I find the work interesting. I think it can be consider to publish if the following issues are solved:

 

1. Figure 2（c and d） clarity needs to be improved

 

2. Is the curve in Fig. 5(b) the degradation performance of ZnO prepared under UV light? (and not without UV light)）

 

3.line50: “temperature(75-95° C)is“ may be ” temperature (75-95° C) is“

 

4. MB: It seems that for MB photodegradation both h⁺ and radical •OH radicals are important. (https://www.sciencedirect.com/science/article/pii/S0926337316308268）

 

Phenol: The degradation efficiency of phenol decreases by 32.5%, 77.6% and 65.2% after adding scavenger, respectively. The contribution order of h⁺,·OH and·O^2- is·OH > ·O^2-> h⁺.（https://www.sciencedirect.com/science/article/pii/S0926337322007809）

 

RhB: while h⁺ and O^2·− radical play a crucial role in photocatalytic reaction for visible light irradiation, except for radical ·OH. ( https://www.sciencedirect.com/science/article/pii/S092633731830002X)

 

According to other literatures, the main active substances for degrading different pollutants are different. Therefore, in Table 1, can more literatures on MB degradation by ZnO be added for comparison.

 

5. The referenced literature does not have the latest research progress, and the latest reference should be appropriately added.

 

6. In this paper, the in-situ synthesis of ZnO in deionized water may be added to the manuscript to compare the performance of in-situ synthesis of ZnO in MB solution to degrade MB.

Author Response

Zn plates were immersed in an aqueous solution of MB, and an in situ ZnO nanostructure growth and UV photocatalytic effect were investigated. One of the significant advantages of this method is that it does not require any chemical additives, the developed materials were characterized by a wide spectrum of various techniques（SEM、EDS、XRD、XPS）which allowed to recognize their properties. I find the work interesting. I think it can be consider to publish if the following issues are solved:

1. Figure 2（c and d）clarity needs to be improved

 We thank the reviewer for this recommendation. We have replaced an image with better clarity.

2. Is the curve in Fig. 5(b) the degradation performance of ZnO prepared under UV light? (and not without UV light)）

 Yes, the curve shows the degradation of MB due to ZnO under UV light. We have changed the figure caption to the following to make it clearer.

“Ln(C₀/C) vs. time plot of the degradation of MB with ZnO nanostructures and UV light”

3.line50: “temperature(75-95° C)is“ may be ” temperature (75-95° C) is“

We have corrected this on this line and everywhere else in the document.

4. MB: It seems that for MB photodegradation both h⁺and radical •OH radicals are important. (https://www.sciencedirect.com/science/article/pii/S0926337316308268）

 Phenol: The degradation efficiency of phenol decreases by 32.5%, 77.6% and 65.2% after adding scavenger, respectively. The contribution order of h⁺,·OH and·O^2- is·OH > ·O^2-> h⁺.（https://www.sciencedirect.com/science/article/pii/S0926337322007809）

RhB: while h⁺ and O^2·− radical play a crucial role in photocatalytic reaction for visible light irradiation, except for radical ·OH. ( https://www.sciencedirect.com/science/article/pii/S092633731830002X)

 According to other literatures, the main active substances for degrading different pollutants are different. Therefore, in Table 1, can more literatures on MB degradation by ZnO be added for comparison.

In Table 1, we wanted to compare ZnO photocatalysts that were synthesized in a similar design (ZnO immobilized on Zn surface). But we could only find one research article which used the same photocatalyst design and tested the degradation of MB. Hence, we included research works, that used the same design, but reported the degradation of other organic dyes. We agree that the readers might be interested in comparing MB degradation performances as well. Hence, we have added some more research articles to Table 1. These articles report the degradation of MB with ZnO, but ZnO is immobilized on other surfaces like glass, silicon, and PET, instead of Zn.

 

5. The referenced literature does not have the latest research progress, and the latest reference should be appropriately added.

We have added some latest research articles to the references.

6. In this paper, the in-situ synthesis of ZnO in deionized water may be added to the manuscript to compare the performance of in-situ synthesis of ZnO in MB solution to degrade MB.

We thank the reviewer for this suggestion. Currently we do not fully understand the growth mechanism of the ZnO nanostructures. For example, we need to investigate if increasing the duration of immersion of the Zn plates in DI water and MB would increase in size of the ZnO nanostructures. The ZnO nanostructures synthesized in DI water may continue to grow when they are immersed in MB to test their photocatalytic performance. Then the comparison might not be accurate. Hence, we have decided to investigate more on the ZnO nanostructure growth in both DI water and MB and then make a comparison as suggested by the reviewer in a future study.

Reviewer 3 Report

Recommendation: Reconsider after major revisions.

Comments:

The author reported, “In situ growth and UV photocatalytic effect of ZnO nanostructures on a Zn plate immersed in methylene blue”. The authors discussed the possible mechanisms of this work. The following comments must be addressed before it is reconsidered in the catalyst journal.

 

1.        Both the abstract and introduction the length should be reconstructed. The paper should have a consistent linguistic and phrasal framework from beginning to end.

2.        Why the ZnO patterns are not visible in XRD patterns. However, XPS spectras showing strong peak. Should show pure ZnO XRD pattern.

3.        Author should perform catalyst dosage at varied concentrations and pH.

4.        Author should perform detection of radical activity using like potassium iodide, ethylenediaminetetraacetic acid, benzophenone, and potassium persulfate scavengers.

5.        How about the reusability of ZnO?

6.        In the product synthesis, the amount of solvent, the company, and purity of the chemicals, are not present. I wonder if other researchers can try to reproduce it

7.        Essentially related work should be compared present work with previously reported photocatalytic degradation of methylene blue summarized in the comparison Table.

8.        The authors did not explain the limitations. Should I assume that there are no limitations? It would be nice if they said the future perspectives and their limitations in conclusion, which can attract more readers.

9.        The typos and grammatical errors are scattered throughout the paper and need to be corrected with the utmost care.

10.    Units:
Use SI units; follow the correct format (e.g., mg kg^-1).
Use M for mol/L; d for days, h for hours, min for minutes, etc.
Be consistent! Use uppercase L for l (litre).

11.     Please always leave a space between number and SI unit (e.g. 4.6 mm, 21 °C, 24 h etc.),
however no space before the "%", "/" and ":" signs. Moreover, provide an adequate space between words.

 

 

 

Author Response

Comments:

The author reported, “In situ growth and UV photocatalytic effect of ZnO nanostructures on a Zn plate immersed in methylene blue”. The authors discussed the possible mechanisms of this work. The following comments must be addressed before it is reconsidered in the catalyst journal.

 

1. Both the abstract and introduction the length should be reconstructed. The paper should have a consistent linguistic and phrasal framework from beginning to end.

We have edited a few sentences in the abstract and believe that it summarizes the manuscript within the 200-word limit. The introduction has been edited to add a few more sentences to emphasize certain points, as recommended by the other reviewers.

The second comment of the reviewer was not clear to us. Can the reviewer elaborate on “consistent linguistic and phrasal framework”? It would be very helpful if we can get some examples that the reviewer thinks we did not follow a consistent linguistic and phrasal framework.

2. Why the ZnO patterns are not visible in XRD patterns. However, XPS spectras showing strong peak. Should show pure ZnO XRD pattern.

XRD profiles of Figure 3(b) and (c) in fact show a ZnO peak at 34.4°. They are low intensity peaks though probably because of the small thickness of the ZnO layer and also due to the presence of amorphous Zn(OH)₂ component, which does not produce XRD peaks. While the depth of information of XRD is in the order of a few to several microns from the sample surfaces, XPS only collects the information (photoelectrons) from within ~10 nm from the surfaces.

3. Author should perform catalyst dosage at varied concentrations and pH.
4. Author should perform detection of radical activity using like potassium iodide, ethylenediaminetetraacetic acid, benzophenone, and potassium persulfate scavengers.
5. How about the reusability of ZnO?

We thank the reviewer for these great suggestions (comments 3-5). However, the purpose of this manuscript is to report the first observation of ZnO nanostructure growth on a Zn surface by its immersion in MB at room temperature and also the simultaneous photocatalytic degradation of MB with UV light. Experiments with different MB concentration, pH and radical scavengers will definitely give more insights about nanostructure growth and the photocatalytic degradation mechanisms. These aspects will be investigated in a future work.

About testing the reusability of the ZnO nanostructures, at this time we have limited knowledge about the nanostructure growth in MB. We do not know if the ZnO nanostructures would keep growing when they are immersed in MB again for testing the reusability. If they continue to grow, the comparison between the 1^st and 2^nd or 3^rd use may not be accurate as the nanostructure morphology might be different at each stage. Hence, we have to investigate more about the ZnO nanostructure growth before we can test the reusability.

 

6. In the product synthesis, the amount of solvent, the company, and purity of the chemicals, are not present. I wonder if other researchers can try to reproduce it

The solvent used here in our experiments is methylene blue (MB). The concentration we used is mentioned in the manuscript (2.5 mg/L). We have added the manufacturer name and purity.

7. Essentially related work should be compared present work with previously reported photocatalytic degradation of methylene bluesummarized in the comparison Table.

We thank the reviewer for this suggestion. In Table 1, we wanted to compare ZnO photocatalysts that were synthesized in a similar design (ZnO immobilized on Zn surface). But we could only find one research article which used the same photocatalyst design and tested the degradation of MB. Hence, we included research works that used the same design, but reported the degradation of other organic dyes. We agree that the readers might be interested in comparing MB degradation performances as well. Hence, we have added some more research articles to Table 1. These articles report the degradation of MB with ZnO, but ZnO is immobilized on other surfaces like glass, silicon, and PET, instead of Zn.

8. The authors did not explain the limitations. Should I assume that there are no limitations? It would be nice if they said the future perspectives and their limitations in conclusion, which can attract more readers.

We thank the reviewer for this great suggestion. We have added the following sentences to the discussion to mention the limitations and future work.

“One of the limitations of this work is that the ZnO nanostructure growth mechanism is not fully understood. In a future study, we will investigate the how the ZnO nanostructure growth is affected by the time of immersion in MB. It is important to understand if the nanostructures continue to grow during longer immersion times, which may alter their photocatalytic performance. We will also investigate the effect of concentration, pH, and type of organic dye on the photocatalytic activity, as well as the photocatalytic degradation mechanism. We also plan to test the durability of the nanostructures against photocorrosion. Understanding the growth mechanism and durability determines the long-term reusability of the ZnO nanostructures which is crucial for photocatalytic water treatment applications.”

9. The typos and grammatical errors are scattered throughout the paper and need to be corrected with the utmost care.
10. Units:
    Use SI units; follow the correct format (e.g., mg kg^-1).
    Use M for mol/L; d for days, h for hours, min for minutes, etc.
    Be consistent! Use uppercase L for l (litre).
11. Please always leave a space between number and SI unit (e.g. 4.6 mm, 21 °C, 24 h etc.),
    however no space before the "%", "/" and ":" signs. Moreover, provide an adequate space between words.

We thank the reviewer for mentioning these errors (comments 9-11). We have reconstructed some sentences and also corrected the typos.

Round 2

Reviewer 1 Report

The authors have performed the revision work in an acceptable manner. Now  this revised manuscript can be published in the  journal of Catalysts

Reviewer 3 Report

Authors have followed almost all recommendations of Reviewers and the article has enhanced its quality. In my opinion the paper should be published in catalyst journal in its current state.