Facile Synthesis, Characterization, and Photocatalytic Activity of Hydrothermally Grown Cu²⁺-Doped ZnO–SnS Nanocomposites for MB Dye Degradation

Round 1

Reviewer 1 Report

The manuscript is devoted to the development of new inorganic photocatalysts based on metal oxides for the oxidation of organic water pollutants. The authors used a hydrothermal method for synthesis of a material based on a ZnO-SnS semiconductor combination doped with copper salts. The authors carefully characterized the obtained material by various methods. The authors studied the kinetics of dye oxidation in an aqueous medium in the presence of this catalyst under the visible light irradiation and found the optimal copper content in the material, which provides maximum activity. The material showed good results, the authors compared it with already described catalysts of this kind and demonstrated the advantage of their catalyst. The material was demonstrated to keep activity in 4 cycles. The work deserves publication.

Notes: 

1) The authors used HIPR-MP400 UV-Vis annular-type photo reactor. it is difficult to find its description in open sourses, the photo and general scheme should be given in the supporting information to provide reproducibility of the results.

2) Slow decreasing of activity was observed after 4 cycles. What is the reason? Could it be leaching of the copper from the material to aquatic media? (It can cause the pollution of the water with Cu-ions). In which form does "Cu²⁺" present in the material?  Does the structure and copper content  of the material change after 4 cycles? These questions should  at least be discussed.

Author Response

We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-1 comments is appended below for your convenience.

Author Response File: Author Response.pdf

Reviewer 2 Report

The hydrothermal approach was used to create Cu2+-doped ZnO-SnS nanocomposites, according to the present work. Various cartelizations are carried out, and the material is employed as a catalyst for the degradation of MB from wastewater with the help of solar irradiation. The work is intriguing for the Catalysts journal, however, it needs extensive modification before it can be published. My thoughts are as follows:

1. Replace "with high purity" in the experimental section with the actual % ratio of purity for each component.
2. What is the water-to-ethanol ratio in the prepared deionized water-ethanol mixture?
3. “Powder samples were dispersed in ethanol solution”, for which measurements?
4. It is necessary to specify the precise intensity obtained by the solution from the solar simulator.
5. It is preferable to report the MB concentration in molar units.
6. At line 165, edit “mixed phase” to “mixed phases”.
7. The peaks associated with the orthorhombic phase are relatively tiny and unclear on the XRD charts. As a result, I believe the XRD is insufficient to demonstrate the occurrence of such a phase.
8. The authors must double-check the structural analysis.
9. What is the ZnO/SnS ratio in ZnO-SnS nanocomposites?
10. At line 172, edit “average crystallite size” to “average crystallite size (D)”.
11. The reference given for equation (3) is incorrect; please offer a more appropriate reference.
12. Figure 2 presents the same data as Table 1, hence it should be eliminated.
13. How do the authors stress the phase ZnO or SnS in Figure 3?
14. The Tauc relation should be added with the appropriate reference, and the optical bandgap transition mechanism type should be identified.
15. The unit of energy (x-axis) should be specified in Figure 5b.
16. Equation (4) requires a reference, and I believe Reference 37 will suffice.
17. Figures 6c and 6d are duplicated for Table 2.
18. Graphene and graphene oxide-based compounds are well recognized for their ability to degrade organic dyes. However, the authors only report a 64 percent efficiency in Table 3. I disagreed with this discussion because great efficiency has been observed elsewhere, for example, https://doi.org/10.1007/s10904-021-02146-3.
19. The conclusion needs to be enhanced.

Author Response

We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-2 comments is appended below for your convenience.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

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Author Response

We appreciate the efforts of the reviewers and Academic Editor for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the Reviewer-2 comments is appended below for your convenience.

Comment 1:    The authors should try to incorporate in the manuscript in more detail their responses to Reviewer#2, as an example in their response described that ''The Abet solar simulator delivered the light with an intensity of 1000 W/m²'' but this information was not added in the manuscript. A reply to Reviewer# second report should be provided.

Response:        We acknowledge the Academic Editor opinion. As suggested, the solar simulator light intensity information was included in the revised manuscript. Thank you very much for your valuable suggestion.

Comment 2:    In addition, for the proposed mechanism there is no experimental evidence but only the use of one reference. The authors should clarify that the proposed mechanism is based on bibliographic data and should use more references for the discussion.

Response:        We acknowledge the Academic Editor's opinion. As suggested, the recent literature related to photocatalytic mechanism was included in the revised manuscript. Thank you very much for your valuable suggestion.

Author Response File: Author Response.pdf