The Influence of Synthesis Methods and Experimental Conditions on the Photocatalytic Properties of SnO₂: A Review

Round 1

Reviewer 1 Report

The file is attached with my comments.

Comments for author File: Comments.pdf

Author Response

On behalf of all authors, I am glad to submit the revised version (R1) of the manuscript catalysts-1623451, and now we hope it attends all the expectations. We thank the Reviewer for all questions and comments that led us to improve the new version. Corrections were made in the manuscript in response to the all the concerns and they are highlighted in red. A detailed response to the remarks is given below.

 

REVIEWER #1

Main remarks

#1: In my opinion the article seems to explain well some fundamentals on semiconductors functionality and to describe a variety of pollutants in water. But their approach makes it very confusing because the authors want to target so many issues and pollutants at once. Specially for the given attention on those pollutants (e.g., PCDD, PCBs, EDCs, etc.) in the introduction and section 2.

Response: We agree with the reviewer. In the revised version of the manuscript, we present a brief discussion about POPs and their classification. However, we draw attention to organic dyes, a type of POP, as the main target pollutant explored to evaluate photocatalytic properties of SnO₂ in literature. So, paragraphs related to PCDD, PCBs, EDCs pollutants, for example, were deleted from the manuscript and paragraphs related to recent advances in photocatalysis of organic dyes using SnO₂-based photocatalysts were included in the revised version. So, we kindly invite the Reviewer to go through the revised version of the manuscript to see the main changes.

 

#2: The authors are also missing the opportunity to accurately target their review and to create better comparative tables and plots (e.g., physicochemical, optical properties, efficiencies, etc.).

Response: This is an important aspect. We have now addressed this point through the creation of new tables, in which different aspects of the photocatalysts and photocatalysis are compared.  Please see: Table 2, Table 3 and Table 4.

#3: The authors’ review feels more like an academic textbook rather than a research review. Furthermore, the manuscript was difficult to follow, and it had a lot of repetitive facts and irrelevant descriptions of pollutants that were not even required in their review.

Response: We appreciate the opportunity to improve this point. The structure of the revised version of the manuscript was modified to improve flow reading. In addition, irrelevant descriptions and repetitive facts were deleted, and some paragraphs or fragments of them were placed elsewhere in the articles to avoid repetition.

 

#4: I believe some additional editing in their formulas, acronyms, and units as well as English proofreading will benefit and improve both the readability and the flow of it.

Response: We appreciate the opportunity to improve the manuscript. We believe that we have addressed this remark as the text was fully revised, which includes English language and formatting (acronyms, formula, units were, and others). 

 

#5: In conclusion, the authors’ work is interesting comparing different synthesis of SnO2 and its effectiveness to degrade mostly dyes pollutants. Nevertheless, at this moment, this article is not suitable for publication, and I would recommend major corrections before it is accepted for its publication on Catalysts (MDPI).

Response: We appreciate the remark! We have made all changes and now we hope it attends the reviewer expectation.

 

General Remarks:

1. Keywords:

• Page 1, line 18. There are numbers after each word, please correct or explain what are those?

            Response: This has been corrected by deleting the numbers after each keyword.

 

2. Introduction

• Overall, the introduction has very long sentences, repetitive use of ideas and lack of connections between paragraphs. This makes the introduction very difficult to follow and to understand the objectives. Please shorten your sentences, simplify, and connect your ideas.

            Response: This was corrected! Long sentences were shortened, ideas were connected, and English language revision was made, so we believe the readability of the introduction is now more adequate.

 

• Page 1, lines 24-27. The paragraph is very confusing and does not make sense. Please, try to clearly explain your initial statement.

            Response: The initial sentence of the introduction has been corrected!

 

• Page 2, line 61. AOP’s stands for advanced oxidation process [1], watch out with the consistency use of acronyms based on the literature or other references.

            Response: We thank the reviewer for the remark! All acronyms based on literature were checked and corrected in the manuscript!

 

• Be aware in the consistency use of subscripts with your formulas. For instance, on page 2, line 78 and line 83 should be TiO2 and SnO2, respectively.

            Response: We thank the reviewer for the remark! Subscripts and superscript have been checked and corrected throughout the manuscript.

 

6) Page 2, lines 82 to 86. References missing to support your arguments about SnO2.

            Response: We thank the reviewer for the remark! Article search have been done using different database that indicated a great number of published articles that pointed out SnO₂ as an important and efficient photocatalyst. In addition, this fact has also been supported by the articles explored and discussed in the review work.

 

3. Persistent Organic Pollutants…

• This section has very interesting facts, however, they need to be shortened and connect better between them.

            Response: We appreciate the remark! We have shortened this section and we believed that now it attends all the reviewer’s expectations

 

• This section is also repetitive in some paragraph ideas. For instance, on the introduction lines 101 to 111 are very similar in context to the ones on lines 23 to 38. These should be omitted or simplify.

            Response: We appreciate the remark! We have shortened this section and repetitive facts we have corrected, deleted and/or placed where was more adequate.

 

• Page 3, lines 112 to 125. I like the idea of explaining the use of the different substances considered as POP’s. But this should be added as appendix for those interested in learning more about those. Otherwise, your review losses the context and gets unclear. The scope of your review should be stronger from the beginning. This will help your readers to understand better your work.

            Response: We appreciate the reviewer for the remark, and we agree with the reviewer! We addressed all these points by changing the structure of the review, by focusing on the recent advanced in photocatalytic efficiency of SnO₂ toward dyes degradation. Finally, we have decided to delete the discussion about other different POPs to avoid confusion and help to readers understand better our work.

 

 

3. Studies proposed by the literature…

10) Very good ideas but very confusing paragraphs. The information is just to find any pollutant and how it was degraded. This article does not have a proper structure and ideas connections. The authors need to work on it.

            Response: We appreciate the reviewer for the remark! We addressed all these points by structuring the review and improving the discussion about recent advances in photocatalysis of organic dyes using SnO₂-based photocatalysts and how synthesis method and conditions change structure, particle size and morphology, texture, electronic defects, bandgap energy and other materials’ characteristics that directly impacted the efficiency of the final product. Therefore, we believe that the review is now more adequate for publication.     

 

• Page 5, line 193. Flow rate units should be L m-1 . The authors should be consistent and careful with the metric units used in the manuscript.

            Response: We thank the reviewer for the remark! All the units were carefully checked and corrected.

 

 

4. Role of catalysts in the efficiency of …

12) Page 6, line 228. Irradiation instead of radiation.

            Response: This was corrected throughout the manuscript.

 

• Page 6, lines 236-243. It is very confusing to follow, and it has again very long sentences.

            Response: This corresponding paragraph was shortened and corrected.

 

14) There is an opened bracket in line 237 with no closed bracket.

            Response: This was corrected!

 

15) Page 6, line 251. OH should be ⦁OH radical.

            Response: This was corrected!

 

16) Page 6, lines 240-243. The authors must maintain the consistency of their acronyms of valence band (VB) and conduction band (CB) rather than (BV) and (BC) in their manuscript.

            Response: This was corrected!

 

17) Table 1 is meaningless they way is presented. It should have a brief description of what the author intended to communicate instead of just throwing information. Furthermore, for a Review is more impactful to create a comparative table of all the presented catalysts with the actual degradation efficiencies, type of reactions, and the most important physical and optical properties.

            Response: We thank the reviewer for the remark! Information in Table 1 was modified and additional information about photocatalysis of dyes using SnO₂-based photocatalysts were included. A description of what Table 1 lists was provided. In addition, Table 2, Table 3, and Table 4 were included in order to compare different synthesis employed to prepare the catalysts, their characteristics, photocatalysis conditions and photocatalytic efficiency.   

 

            18) Overall, in this section, the authors are missing consistency to add relevant information when describing other authors’ work. For instance, they must include the following: the type of reactor used, the type of light source irradiated, the photocatalyst load, the catalyst concentration and the initial pollutant concentration. The more information the better. This comment is also related to the previous point 17 and the importance to create a comparative table with all this information.

            Response: We thank the reviewer for the remark! Important information on Table 2, Table 3, and Table 4 have been. In addition, discussion about other relevant works have also been included (Please see sections 4.1, 4.2 and 4.3).

   

19) Captions should be a bit more descriptive. Figure 2 papers published of what? During the 2017- 2021 period.

            Response: We thank the reviewer for the remark! Figure and table captions have been modified as requested.

 

20) What is the conclusion of this section and the opinion of the authors, how this section led the authors to select tin oxide for further studies? This is not clear at all.

            Response: We appreciate the remark. Conclusions about reported works have been made in each topic of the review. In addition, in the end of last section and before Conclusions and Final remarks we included paragraphs to summarize and conclude the findings of the works.

Please see, for example: Most of the authors reported that the amount of materials for the formation of the SnO₂ with different particle size and morphology, besides doped-SnO₂ with an appropriate amount and type of dopant and also the formation of composite with SnO₂ play an important role in improving the photocatalytic activity of SnO₂ material. In relation to composites, the excess of both species can be harmful to contact surface between the phases, mainly due to the high degree of particles agglomeration. Tests using scavengers, such as p-benzoquinone (BZ, C₆H₄O₂), isopropanol (ISO, (CH₃)₂CHOH), and ammonium oxalate monohydrate (AO, (NH₄)₂C₂O₄·H₂O) indicate •OH are the main species in most photocatalytic mechanism. However, to obtain more insights about the photocatalytic mechanism involved in composite materials, we seek to understand the charge transfer between the phases from the band structures of each individual material. Structural and electronic defects can also generate energy levels between the VB and CB, and, therefore, modify the photocatalytic mechanism of composites. The creation of different interfaces between the phases may reduce charge carriers’ recombination, leading to the formation of great number of free radicals to improve photocatalysis. In addition, several other parameters can impact the photocatalytic efficiency of composites, such as phase composition, surface area, morphology, particle size, pore structure, electron-hole recombination rate and band gap energy of the individual components. Some authors showed that the high surface area and the presence of pores are more effective parameters that affect dye degradation since the existence of several active sites, responsible for the adsorption of molecules, are crucial for the photocatalysis to occur.

            Based on the findings above, it can be concluded that to design a new photocatalytic material with specific characteristic, one has to consider optimizing type and amount of dopants and interface characteristics between materials, or even the nature of the desired product (powder, film, etc), besides the microstructure of the material (particle size and morphology) by a choice an specific synthesis methodology and appropriate experimental conditions.

 

 

5. Tin Oxide structural characteristics…

21) Page 9, line 369. Correct all the formulas and oxidation valences with proper subscripts and superscripts. For instance, Sn4+ should be Sn4+.

            Response: Appropriate corrections have been made throughout the manuscript.

 

22) Page 9, line 374. Correct citation style. 6. Synthesis methods and influence of experimental…

            Response: We thank the reviewer for the remark! Correction on citation style has been made.

 

23) Table 2 is missing to explain better the experimental conditions. For instance, the temperature of what stage? time is referred to what? preparation, calcination, drying, agitation etc. 3

            Response: We thank the reviewer for the remark! Table 2 has been updated and new important information has been included.

 

24) Page 11, lines 422-425. Check in this section for consistency in subscripts and superscripts used in equations and numerical units.

            Response: All these inconsistencies have been corrected throughout the manuscript

 

25) The authors should be consistent in their review. In some cases, authors explain well the method, and the operation process, in addition to presenting relevant results, this is the case of lines 412- 425. Unlike other paragraphs you only mention the bare minimum.

            Response: We appreciate the remark! We have addressed these points throughout the manuscript by improving the discussion of the important aspects reported in the articles.

 

26) Conclusions of this section are inexistent. The authors need to mention about what they believe the best method in terms of their observations.

            Response: We appreciate the remark! We have addressed these points by inclusion important aspects about the findings of the different articles explored in the review. In addition, we have included critical opinions about each article in the sections concerning photocatalytic efficiency of SnO₂-based photocatalysts reported in literature.

 

7. Summary

27) What are the overall conclusions of this work? It is important to emphasize the relevant statements of each section based on authors expertise and opinion.

            Response: We appreciate the remark! Appropriate correction was made.

            Please see Conclusions and Final remarks:

            From this review work, it was possible to evaluate the importance of synthesis methods and experimental parameters in obtaining tin oxide-based materials with high performance in heterogenous photocatalysis of persistent organic pollutants, more specifically, organic dyes. The search for new materials and methodology that provide efficient results for the remediation of such pollutants has been one of the great challenges for the scientific community. Among the studied promising materials, SnO₂ has shown excellent results as a catalyst in heterogeneous photocatalysis processes due to its intrinsic characteristics, which have been responsible for the material's conductivity, optical and electrical properties, and high thermal stability. As a consequence of the choice of synthesis methods and experimental conditions, it was possible to evidence different the morphology, particle size, surface area, structural modifications, optical bandgap energy, surface and bulk defects and, therefore, excellent results in the application of pure and modified SnO₂ toward the degradation of persistent organic pollutants (POPs), showing a degradation efficiency of 90-94 % for a series of dyes.

            Considering pure SnO₂ catalysts, synthesis conditions may influence, especially, particle size and morphology, specific surface area, crystallinity, and the presence of electronic defects on surface and bulk of the materials, which are important parameters to change photocatalytic efficiency under both UV and visible irradiation. With respect of doped SnO2, type and amount of dopants may introduce different intermediate levels within the gap, decreasing band gap energy to improvement of the photoabsorption in visible light. Finally, it has been demonstrated that the composited formed with SnO2 is responsible for band structure alignment and improvement of charge separation that led to an increased photocatalytic activity when compared to the individual components. It is still important to highlight that, the study of the reaction mechanism involved in the dye degradation is an important aspect, which allow to design new and efficient SnO₂-based photocatalyst, understanding their laminations in order to use them in practical devices.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The present review proposed a structure-activity relationship of tin oxide (SnO₂) towards advanced photocatalytic oxidative processes for environmental pollutants removal. The manuscript and proposed review are somewhat important in advancing the high performance and low-cost photocatalyst, especially for those working in developing countries where environmental pollutants in water bodies are a huge challenge. However, there are some missing points that make the manuscript still lack novelty. My suggestions to make the manuscript meets the high quality and publishable in Catalyst are as follows:

1. In the introduction, authors are suggested to include previous related or similar review papers (such as https://www.sciencedirect.com/science/article/pii/S1385894721031454, https://doi.org/10.1016/j.psep.2017.01.022, etc.) and please justify/highlight the novelty of the current review. This will help the readers to understand the importance of the presented review.
2. Still, in the introduction, several methods to synthesize SnO2 were mentioned such as hydrothermal, chemical precipitation, polymeric precursor, sol gel, etc. But the solvothermal is also an effective method to control the morphology of SnO₂.https://doi.org/10.1016/j.ceramint.2019.05.043, https://doi.org/10.1016/j.jmst.2020.02.041
3. In Figure 1, authors are suggested to include the year when data is collected. A similar problem with Figure 3. The authors should have a permission/copyright clearance of the used images from the corresponding publisher.
4. For the section 4 (Role of catalysts in the efficiency of heterogeneous photocatalysis process), a schematic illustration will help the reader to understand the particular topic better. So, I suggest making illustrations to explain the Advanced oxidative processes (AOP's) over the semiconductor, especially the SnO2.
5. From page 7 onwards, the authors listed recent works on different strategies to improve the photoinduced oxidative processes of SnO₂ such as morphology design, heterostructures, doping, etc. However, the underlying oxidative mechanisms of the modified SnO₂ are missing.
6. Please adjust the reference format according to Journal guidelines. I found a problem with section 5 reference ([LAVANYA et al., 2015; SAKTHIRAJ BALACHANDRAKUMA, 2015; DAS et al., 2018] that is different with whole formats.
7. Please check the spelling of Figure in Line 384.
8. I found some misspellings throughout the manuscript, please always double-check before the submission.
9. The review papers should provide the outlook, future challenges, or similar from the author's knowledge and perspectives after the summary. Please provide this content, otherwise, the review manuscript can not be published.
10. Similarly, the brief structures should also be stated in the abstract.

Comments for author File: Comments.pdf

Author Response

On behalf of all authors, I am glad to submit the revised version (R1) of the manuscript catalysts-1623451, and now we hope it attends all the expectations. We thank the Reviewer for all questions and comments that led us to improve the new version. Corrections were made in the manuscript in response to the all the concerns and they are highlighted in red. A detailed response to the remarks is given below.

 

REVIEWER #2

Main remarks The present review proposed a structure-activity relationship of tin oxide (SnO2) towards advanced photocatalytic oxidative processes for environmental pollutants removal. The manuscript and proposed review are somewhat important in advancing the high performance and low cost photocatalysts especially for those working in developing countries where environmental pollutants in water bodies is a huge challenge. However, there are some missing points that make the manuscript still lack novelty. My suggestions to make the manuscript meets the high quality and publishable in Catalyst are as follows:

 

Remark #1. In the introduction, authors are suggested to include previous related or similar review papers (such as https://www.sciencedirect.com/science/article/pii/S1385894721031454, https://doi.org/10.1016/j.psep.2017.01.022 , etc.) and please justify/highlight the novelty of the current review. This will help the readers to understand the importance of presented review.

            Response: This is an important aspect. A comparison of these previous reviews with the current one has been made by the inclusion of paragraphs in inclusion of the following paragraphs:

            Please see Introduction section: Some review articles have dealt on the applications of SnO₂ materials in photocatalysis. However, up to our knowledge, none of them draw attention to particular characteristics of synthesis method and experimental conditions used to prepare the different SnO₂-based photocatalysts and how these directly impacted on the sample characteristics, properties and photocatalytic efficiency for organic dyes degradation. For instance, in the review published by Al-Hamdi et al. [48], results from studies conducted up to 2017 were shown, which focused on the applications of SnO₂ in advanced oxidative processes for the degradation of organic pollutants, such as phenols, phthalates, and other toxins in water. In addition, authors listed relevant investigations on fundamental aspects related to SnO₂, such with structure and properties, charge transfer mechanism involved and parameters that affect the photodegradation of pollutants in aqueous solutions, such as catalyst load, concentration of the contaminant and pH. In a recent review, Sun et al. [49] discussed the importance of employing SnO₂-based photocatalysis in ​​environmental science and energy fields. The authors focused on several studies dealing with strategies used to enhance the SnO₂ properties for photocatalysis. The effect of doping, formation of solid solutions, stoichiometry, particle size and morphology, besides the formation of hierarchical, porous and heterojunctions structures has been demonstrated. Apart from the applications in water splitting and organic dyes photodegradation, the authors discuss gave examples of SnO₂ applications in Cr(VI) and CO₂ reduction. The authors also mentioned the complexities of applying photocatalysts in large scale to simulate the real scope of the industry since, although significant progress has been achieved in the improvement of the photocatalytic efficiency of SnO₂, there are still major challenges to be faced.

            Based on what was mentioned, the present work aims to address important aspects about recent and relevant achievements of how the choice of appropriate synthesis method and experimental parameters may influence the morphology, particle size, bandgap energy and, consequently, impact on the photocatalytic properties of SnO₂ for the degradation of different organic dyes as targets persistent organic pollutants (POPs) molecules. Apart from the applications of pure SnO₂ toward the degradation of dyes, studies concerning photocatalysis using metal and nonmental-doped SnO₂ as well as SnO₂-based composites are also summarized. We believe that this literature review may provide important aspects for the better development of SnO₂-based catalysts, understanding the limitations, in order to use them in practical devices.

 

Remark #2. Still in the introduction, several methods to synthesize SnO2 were mentioned such as hydrothermal, chemical precipitation, polymeric precursor, sol gel, etc. But the solvothermal is also an effective method to control the morphology of SnO2. https://doi.org/10.1016/j.ceramint.2019.05.043, https://doi.org/10.1016/j.jmst.2020.02.041

            Response: We agree with the reviewer. Appropriate citation has been made and these two references have been included in the introduction.

            Please see in Section 4: Among the great variety of methods known in the literature, SnO₂-based photocatalysts have been obtained, in their powder form, by solvothermal reaction [87,88], microwave-assisted hydrothermal method [89], chemical precipitation [69], polymeric precursor method [90] and sol gel [14] are mostly used.

See citation in:

[87] Hermawan, A.; Asakura, Y.; Inada, M. and ShuYin. One-step synthesis of micro-/mesoporous SnO₂ spheres by solvothermal method for toluene gas sensor. Ceramics International. 2019,45, 15435-15444.

[88] Hermawan, A.; Asakura, Y.; Inada, M. and Yin, S. A facile method for preparation of uniformly decorated-spherical SnO₂ by CuO nanoparticles for highly responsive toluene detection at high temperature. Journal of Materials Science & Technology. 2020, 51, 119-129.

 

Remark #3. In Figure 1, authors are suggested to include the year when data is collected. A similar problem with Figure 3. The authors should have a permission / copy right clearance of the used images from the corresponding publisher.

            Response: We agree with the reviewer. Appropriate changes were made in all the Figure and Table captions in order to give a better description of what we intended to present. In addition, we have constructed an image to illustrate the crystal structure of SnO₂ by VESTA software using structural parameters reported in literature

            Please see Captions of Figure 2, Table 1 and Figure 3.

            Please see: Figure 4 and its corresponding caption.

 

Remark #4. For the section 4 (Role of catalysts in the efficiency of heterogeneous photocatalysis process), a schematic illustration will help the reader to understand the particular topic better. So, I suggest to make illustration to explain the Advanced oxidative processes (AOP's) over the semiconductor especially the SnO2.

            Response: We agree with the reviewer. We have addressed this point through the addition of a paragraph explaining the charge generation and transfer during the photocatalyst and an illustration showing such mechanisms has been provided as requested. 

            Please see Page 1: During photocatalysis, photogenerated e^-/h⁺ can migrate to the surface of the material and interact with adsorbed species; in addition, they can be captured in intermediate energy states, or undergo recombination. When e- are in BC, one of the very important reactions can occur, which is the reduction of the adsorbed O² on the catalyst surface to superoxide (^•O^2-) radicals, avoiding recombination of electrons and the hole. This results in the accumulation of oxygen radical species that can participate to photocatalytic reactions. On the other hand, when h+ from the VB migrate to the catalyst surface it can react with adsorbed H‑O to generate hydroxyl (^•OH) radicals, reacting with the pollutants to be degraded. It is worth mentioning that electronic recombination does not favor photocatalysis, because, if the e^- and h⁺ recombine, the formation of the photogenerated e-/h+ pairs fail to participate in the oxidation-reduction process and formation of free radicals in the process [31-33]. An illustration of the general mechanism involved in photocatalysis is given in Figure 1 as also reported in [31].

            Please see Figure 1:

Figure 1 – General mechanism of the heterogeneous photocatalysis using a hypothetical semiconductor.

 

Remark #5. From the page 7 onwards, authors listed recent works on different strategies to improve the photoinduced oxidative processes of SnO2 such as morphology design, heterostructures, doping, etc. However, the underlying oxidative mechanisms of the modified SnO2 are missing.

            Response: We appreciate the reviewer for the remark. Explanation about the oxidative mechanism and the related reactions have been provided in the section 4.1. We have shown two different mechanisms proposed by different authors for SnO₂ catalyst.

           

Please see mechanisms (Eq (1)-(6)) proposed by Abdelkader et al. [93]:

SnO₂ + UVA(365 nm) → n – SnO₂ (h⁺_(VB) + e^-_(CB))                  (1)

Sn⁴⁺ + e^-_(CB) → Sn²⁺                         (2)

Sn²⁺ + 2O₂ → 2O₂^•- + Sn⁴⁺                 (3)

H₂O + h⁺_(VB) → ^•OH                  (4)

R + h⁺_(VB) → R^•+                                   (5)

(O₂^•-, ^•OH, R^•+) + CR dye → intermediate products → Dye degradation             (6)

 

            Please see also mechanisms (Eq (7)-(12)) proposed by Najjar et al. [97]:

SnO₂ + hυ → SnO₂ + (h⁺_(VB) + e^-_(CB))                                    (7)

SnO₂ (h⁺_(CB)) + H₂O → SnO₂ + H⁺ + ^•OH                       (8)

SnO₂ (e^-_(CB)) + O₂ → SnO₂ + O₂^-•                       (9)

O₂^-• + H⁺ → HO₂^•                                                                                          (10)

HO₂^• + H₂O → H₂O₂ + ^•OH                              (11)

Dye + ^•OH → degradation dye                         (12)

 

Remark #6. Please adjust the reference format according to Journal guidelines. I found problem with section 5 reference ([LAVANYA et al., 2015; SAKTHIRAJ BALACHANDRAKUMA, 2015; DAS et al., 2018] that is different with whole formats.

            Response: This has been corrected!

 

Remark #7. Please check the spelling of Figure in Line 384.

            Response: This has been corrected!

 

Remark #8. I found some misspelling throughout the manuscript, please always double check before the submission.

            Response: We appreciate the remark! We have gone through the manuscript and carefully checked and corrected writing and other minor issues.

 

Remark #9. The review papers should provide the outlook, future challenges, or similar from the authors knowledge and perspectives after the summary. Please provide this content, otherwise the review manuscript can not be published. 10. Similarly, the brief structures should also be stated in the abstract.

Response: We appreciate the remark! Appropriate correction was made.

            Please see Conclusions and Final remarks:

            From this review work, it was possible to evaluate the importance of synthesis methods and experimental parameters in obtaining tin oxide-based materials with high performance in heterogenous photocatalysis of persistent organic pollutants, more specifically, organic dyes. The search for new materials and methodology that provide efficient results for the remediation of such pollutants has been one of the great challenges for the scientific community. Among the studied promising materials, SnO₂ has shown excellent results as a catalyst in heterogeneous photocatalysis processes due to its intrinsic characteristics, which have been responsible for the material's conductivity, optical and electrical properties, and high thermal stability. As a consequence of the choice of synthesis methods and experimental conditions, it was possible to evidence different the morphology, particle size, surface area, structural modifications, optical bandgap energy, surface and bulk defects and, therefore, excellent results in the application of pure and modified SnO₂ toward the degradation of persistent organic pollutants (POPs), showing a degradation efficiency of 90-94 % for a series of dyes.

            Considering pure SnO₂ catalysts, synthesis conditions may influence, especially, particle size and morphology, specific surface area, crystallinity, and the presence of electronic defects on surface and bulk of the materials, which are important parameters to change photocatalytic efficiency under both UV and visible irradiation. With respect of doped SnO2, type and amount of dopants may introduce different intermediate levels within the gap, decreasing band gap energy to improvement of the photoabsorption in visible light. Finally, it has been demonstrated that the composited formed with SnO2 is responsible for band structure alignment and improvement of charge separation that led to an increased photocatalytic activity when compared to the individual components. It is still important to highlight that, the study of the reaction mechanism involved in the dye degradation is an important aspect, which allow to design new and efficient SnO₂-based photocatalyst, understanding their laminations in order to use them in practical devices.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments are attached.

Comments for author File: Comments.pdf

Author Response

On behalf of all authors, I am glad to submit the revised version (R1) of the manuscript catalysts-1623451, and now we hope it attends all the expectations. We thank the Reviewer for all questions and comments that led us to improve the new version. Corrections were made in the manuscript in response to the all the concerns and they are highlighted in red. A detailed response to the remarks is given below.

 

REVIEWER #3

General Remark: This manuscript is a review about the influence of synthesis methods and experimental conditions on the photocatalytic properties of SnO2. In general it can be seen that the manuscript is very poorly written and would need extensive re-writing before it could be considered for publication. Overall, the general impression is that the article is written in a rush, with many too long and confuse phrases and with many grammatical mistakes.

           

Examples: Abstract - long phrase: “SnO2 is an n-type semiconductor with …. and environmental decontamination”.

            Response: We appreciate the reviewer for the concern. The manuscript has been fully revised and re-written appropriately according to the reviewer comments and suggestion. We invite the reviewer to go through the manuscript and see all changes made throughout the text. We hope that now it attends all the reviewer expectations. and the manuscript

In relation to the long phrases, they have been corrected as requested. For instance, please see the example given by the reviewer in the Abstract: SnO₂ is an n-type semiconductor with a band gap between 3.6 and 4.0 eV, whose intrinsic characteristics are responsible for its electrical conductivity, good optical characteristics, high thermal stability, and others. Such characteristics have provided excellent results in advanced oxidative processes, i.e., heterogeneous photocatalysis application. This process involves semiconductors in the production of hydroxyl radicals via activation by light absorption and it is considered as an emerging and promising technology for wastewater.

Remark #1

2. Persistent organic pollutants (POPs) and impacts on water bodies

Page 3

Long and confuse phrases: “Among these contaminants, … to the environment [57]”.

“According to Montagner et al., (2017) [59], in Brazil, … distributed to the population from the same pond”

            Response: As a consequence of the suggested revision and appropriate corrections, these phrases have been deleted from the manuscript.

           

Remark #2

Page 4

 “Norra G. and Radjenovic J. (2021) [64] recently used a … toxic halogenated byproducts”.

 “Among the contaminants analyzed, DEET showed the greatest synergy, … and perchlorate concentrations, respectively”.

            Response: As suggested by the other reviwers, suggested revision and appropriate corrections and this part has been deleted from the manuscript.

 

Remark #3

Page 6

 “In a photochemical process, fillers can react … through the oxidation of H2O or OH- through the hole.”

            Response: As a consequence of the suggested revision and appropriate corrections, this fragment has been deleted from the manuscript.

            A change in the paragraph has been made, please see Introduction: During photocatalysis, photogenerated e^-/h⁺ can migrate to the surface of the material and interact with adsorbed species; in addition, they can be captured in intermediate energy states, or undergo recombination. When e^- are in BC, one of the very important reactions can occur, which is the reduction of the adsorbed O₂ on the catalyst surface to superoxide (^•O₂^-) radicals, avoiding recombination of electrons and the hole. This results in the accumulation of oxygen radical species that can participate to photocatalytic reactions. On the other hand, when h⁺ from the VB migrate to the catalyst surface it can react with adsorbed H₂O to generate hydroxyl (^•OH) radicals, reacting with the pollutants to be degraded. It is worth mentioning that electronic recombination does not favor photocatalysis, because, if the e^- and h⁺ recombine, the formation of the photogenerated e^-/h⁺ pairs fail to participate in the oxidation-reduction process and formation of free radicals in the process [31-33]. An illustration of the general mechanism involved in photocatalysis is given in Figure 1 as also reported in [31].

 

Remark #4

Page 8

 “Das et al. (2018)[41] used Sn/SnO2 heterostructure … to SnO2 in order to reach Fermi level equilibrium”.

            Response: This has been corrected.

            Please see new paragraph related to the study: Das et al. [37] prepared Sn/SnO₂ nanocomposites by precipitation method followed by carbothermal reduction and calcination at 800 ºC for 2h. The authors investigated the photocatalytic property of Sn/SnO₂ composites in the degradation of methylene blue under UV irradiation using a catalyst concentration of 0.5 g L^-1. It was found a maximum efficiency of 41% for pure SnO₂ after 210 min under irradiation, while Sn/SnO₂ composite showed a higher photocatalytic activity of 99%. The highest efficiency observed for the composite was related to the role of Sn on the surface of SnO₂ nanoparticles. As the Fermi energy level of Sn is higher than that observed for SnO₂ due to its lower work function, so, when metallic Sn is bound on the surface of SnO₂ nanoparticles, electrons migrate from Sn to SnO₂ to reach Fermi level equilibrium. The effect of the pH solution on the photocatalytic efficiency of the composites was also evaluated. The pH had a direct influence on the photocatalytic process, being the neutral pH favorable for the degradation of MB dye. Finally, the authors investigated the reusability of the composites after three cycles and confirmed that the photocatalyst is stable, but gradual loss in efficiency was observed due to the loss of the material during recovery processes.

 

Remark #5

Page 11

“Assis et al., (2018) [57] used the method of … in addition to favoring the recovery of the material after use.”

“ Was also observed that the oxide …. after 70 min of irradiation”.

            Response: These fragments have been corrected.

            Please see: Assis et al. [59] used a polymeric precursors method to prepare SnO₂ particles at different temperatures (700, 800 and 900 °C). After being prepared, the powders were impregnated in polystyrene foams in order to increase surface area due to the porous characteristic of polystyrene, besides favoring the recovery of the material after use. The photocatalytic property of the samples was investigated in the degradation of RhB dye under UV irradiation with a catalyst/dye concentration of 0.4 g L^-1. The authors observed, using high resolution transmission electron microscopy (HRTEM), that the SnO₂ samples present nanoparticles with size ranging between 20 and 80 nm. Moreover, the formation of agglomerates was observed in the samples calcined at higher temperatures (800 and 900 ºC). The oxide obtained at lower temperature presented a smaller particle size and a larger surface area, which resulted in a greater photocatalytic activity, degrading 98.2% of degradation of the rhodamine RhB after 70min.

 

Remark #6

            And many others examples in pages 12-14.

Response: The long phrases have been corrected and appropriate changes have been made throughout the manuscript in other to improve readability.

 

 

Remark #7

Summary

“As for the influence of experimental methods … 90-94 % for a series of dyes.”

            Response: The long phrases have been corrected and appropriate changes have been made throughout the manuscript in other to improve readability.

            Please see the corresponding fragment in Conclusions and Final remarks: As a consequence of the choice of the synthesis method and experimental conditions, it was possible to evidence different the morphology, particle size, surface area, structural modifications, optical bandgap energy, surface and bulk defects and, therefore, excellent results in the application of pure and modified SnO₂ catalysts toward the degradation of persistent organic pollutants (POPs). In general, SnO₂-based photocatalysts have shown promising efficiency for degrading a series of different organic dyes

 

Remark #8

- The way the bibliography is presented must be unitary throughout the entire manuscript and not in 3-4 different ways:

 “Haq et al., (2020) [91] synthesized …”

“[92,93,94].” “[LAVANYA et al., 2015;SAKTHIRAJ, BALACHANDRAKUMA, 2015; DAS et al., 2018].”

            Response: This has been corrected! Please see the bibliography format.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have made great work with this new manuscript. There are, however, minor editing details that need to be corrected. The number of pages should be in numerical order. There is also a small problem on page 28 (where it is supposed to be) lines 214-217, the subscripts in the chemical equations should be placed correctly.

In my opinion, the authors have made major improvements in this work and this article should be accepted for publications in Catalysts.

Reviewer 2 Report

authors have made an appropriate change in the manuscript. It is enough for publication

Reviewer 3 Report

Comments are attached

Comments for author File: Comments.docx