Solid State Nanostructured Metal Oxides as Photocatalysts and Their Application in Pollutant Degradation: A Review
Round 1
Reviewer 1 Report
The purpose of the review is presence of application of nanostructured metal oxides (photocatalysts) in pollutant degradation.
The authors did a lot of work, however, I have a few comments:
1) I have objections to the tables. Most of them are based on only one source. Of course, the author mentioned it. But what is the point of copying tables from other articles?
2) Table 2, page 5 - Other similar organic water pollutants malachite green - the table does not clearly show the ends of words.
3) Equation 1 on page 6 - no abbreviations explained.
4) Table 9 - Fe2O3 – should be Fe2O3
5) Page 16, line 373 - Chitosan●(NiCl2●6H2O )x - unnecessary space before brackets
6) All in all, the information contained in this review are too general. I realize this is a literature review. But too many topics are covered without going into them.
Author Response
Referee #1
Comments and Suggestions for Authors
The purpose of the review is presence of application of nanostructured metal oxides (photocatalysts) in pollutant degradation.
The authors did a lot of work, however, I have a few comments:
- I have objections to the tables. Most of them are based on only one source. Of course, the author mentioned it. But what is the point of copying tables from other articles?
Answer: Copying tables from other articles (actually adapted from others) has the purpose of explaining some conclusion that can only be clear if all the content shown in them, is observed. For example, looking at and analyzing table 2, you can add the phrase " For instance, the dependence of the catalytic effectiveness of TiO2 on the preparation method is illustrated in Table 2”
- Table 2, page 5 - Other similar organic water pollutants malachite green - the table does not clearly show the ends of words.
Answer: As referee requested the the correction was made
3) Equation 1 on page 6 - no abbreviations explained. As referee requested all terms of the equation were specified.
4) Table 9 - Fe2O3 – should be Fe2O3
As referee requested the correction was made.
5) Page 16, line 373 - Chitosan●(NiCl2●6H2O )x - unnecessary space before brackets.
Answer: As referee requested the correction was made.
6) All in all, the information contained in this review are too general. I realize this is a literature review. But too many topics are covered without going into them.
Answer: 2 As a Review, it is normal for it to be a review of the literature. As expected, in some topics that belong to the authors, the discussion goes deeper. For example, from page 10 to 21.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This review paper provides an intriguing overview of preparations of nanostructured metal oxide semiconductors through a solid-state pyrolysis method for the degradation of dyes in waters. Based on the thermal treatment of Chitosan●MXn and PS-co-4-PVP● MXn precursors, it is easy to afford pure phase nanostructured metal oxides MxOy, such as TiO2, Fe2O3, NiO, ReO3, IrO2, Rh2O3, Rh/RhO2, and the actinide ThO2. As-prepared metal oxides generally are efficient photocatalysts and often more efficient than the respective MxOy obtained by other solution methods. Given that the motif is not in detailed commented on in previous review papers, I think this review should publish after major modifications as following.
1. The format of this review paper (in term of the research paper sequence, 1. Introduction, 2. Materials and Methods, 3. Results, 4. Concluding) was in the wrong. It should be modified into the regular format of the review paper, focusing on the metal oxide semiconductors prepared by solid-state pyrolysis method for the photo-degradation of dyes pollutants.
2. In the introduction, the content of about Table 2~5 should be moved to the text part for the specific comparison to that of respective case prepared from pyrolysis.
3. A recent review papers (Dongge Ma, Jundan Li, Anan Liu, Chuncheng Chen, Carbon Gels-Modified TiO2: Promising Materials for Photocatalysis Applications, Materials, 2020, 13, 1734; Yuhang Qian and Dongge Ma, Covalent Organic Frameworks: New Materials Platform for Photocatalytic Degradation of Aqueous Pollutants, Materials, 2021, 14, 5600.) should be cited in the revised MS for deeply understanding the photocatalytic degradation mechanism.
Author Response
Referee #2
Comments and Suggestions for Authors
This review paper provides an intriguing overview of preparations of nanostructured metal oxide semiconductors through a solid-state pyrolysis method for the degradation of dyes in waters. Based on the thermal treatment of Chitosan●MXn and PS-co-4-PVP● MXn precursors, it is easy to afford pure phase nanostructured metal oxides MxOy, such as TiO2, Fe2O3, NiO, ReO3, IrO2, Rh2O3, Rh/RhO2, and the actinide ThO2. As-prepared metal oxides generally are efficient photocatalysts and often more efficient than the respective MxOy obtained by other solution methods. Given that the motif is not in detailed commented on in previous review papers, I think this review should publish after major modifications as following.
- The format of this review paper (in term of the research paper sequence, 1. Introduction, 2. Materials and Methods, 3. Results, 4. Concluding) was in the wrong. It should be modified into the regular format of the review paper, focusing on the metal oxide semiconductors prepared by solid-state pyrolysis method for the photo-degradation of dyes pollutants.
Answer: The format was made according to the guidelines given in the guide to the authors.
In relation to the focusing on the metal semiconductors, of the 21 pages of article 11 are dedicated to the authors' own results on the subject, emphasizing the results of photocatalysis. Because there is already a review (Carlos Diaz, Maria Luisa Valenzuela and Miguel Á. Laguna-Bercero Solid-State Preparation of Metal and Metal Oxides Nanostructures and Their Application in Environmental Remediation Int. J. Mol. Sci. 2022, 23, 1093-1122) where the preparation of these nanostructured metal oxides is emphasized, in this review the aspects of photocatalysis are emphasized. In addition, the rest of the review is important in the sense that it discussed the general framework of metal oxides as photocatalyst in the elimination of contaminants such as organic dyes.
- In the introduction, the content of about Table 2~5 should be moved to the text part for the specific comparison to that of respective case prepared from pyrolysis.
Answer: Except for table 2, we think that the transfer of the tables does not proceed. In fact, tables 3-5 deal with data of the type MxOy/M´ nanocomposites, which are absent in our results. Table 2 was presented and discussed in a more general context.
- A recent review papers (Dongge Ma, Jundan Li, Anan Liu, Chuncheng Chen, Carbon Gels-Modified TiO2: Promising Materials for Photocatalysis Applications, Materials, 2020, 13, 1734; Yuhang Qian and Dongge Ma, Covalent Organic Frameworks: New Materials Platform for Photocatalytic Degradation of Aqueous Pollutants, Materials, 2021, 14, 5600.) should be cited in the revised MS for deeply understanding the photocatalytic degradation mechanism.
Answer: As referee requested the references [83, 84] were added and a sentence commenting on these articles was added on page 21 line 522
- In the introduction, the content of about Table 2-5 should be moved to the text part for the specific comparison to that of respective case prepared from pyrolysis.
Answer: Except for table 2, we think that the transfer of the tables does not proceed. In fact, tables 3-5 deal with data of the type MxOy/M´ nanocomposites, which are absent in our results. Table 2 was presented and discussed in a more general context
Author Response File: Author Response.docx
Reviewer 3 Report
The paper gives an overview of nanostructured metal oxides for water remediation. The subject of the paper is interesting, but the paper is sloppy written. The manuscript gives a lot of space to the research done by the Authors, instead more indications on emerging trends or recent developments should be presented, which will be of significant general interest to the journal's wide international readership. Therefore, I recommend major revisions.
General comments:
Include more points related about the difference between catalysts prepared by sol-gel and solid-state methods.
The title needs to be more focused, for example by including a reference to the preparation method via solid state.
Main comments:
1. line 50: “Among the various physical, chemical, and biological technologies used in pollution control, advanced oxidation processes such as photocatalysis are being increasingly adopted in the destruction of organic contaminants due to their high efficiency, simplicity, good reproducibility, and ease of handling [8]”. For large scale municipal applications, current photocatalytic water treatment systems are less attractive because they are more time consuming and have higher costs than other existing advanced oxidation techniques such as UV/H2O2, O3/H2O2, and UV/O3 technologies (see for example: Von Gunten, U. Oxidation Processes in Water Treatment: Are We on Track? Environ. Sci. Technol. 2018, 52, 5062–5075, doi:10.1021/acs.est.8b00586. Cates, E.L. Photocatalytic Water Treatment: So Where Are We Going with This? Environ. Sci. Technol. 2017, 51, 757–758, doi:10.1021/acs.est.6b06035).
The Authors should include recently reported applications to prove that photocatalysis is being increasingly adopted.
2. line 68: The generation of “green hydrogen” is not an environmental remediation process.
3. Check fig. 1: which is the meaning of the arrow with “photocatalytic study”?
4. line 100: “… release the hv energy…”? This sentence is unclear.
5. line 110: “… energy exceeding the band gap…” should read “…energy that matches or exceeds the band gap energy of the semiconductor…”.
6. Table 1. 3rd column on the top “a,b” refer to what? For each material listed in the table 1 reference must be present.
7. Table 2. For each material listed in the table reference must be present. Moreover, in the 5th column “discoloration rate “ should be “ removal %”.
8. Equation 1: Authors must specify the meaning of all terms in the equation.
9. line 159: Table 4 is referred to in the text before Table 3.
10. Table 3 For each material listed in the table reference must be present.
11. Table 4 For each material listed in the table reference must be present. Moreover, for each material specify the removal time.
12. Tables 6 and 7. Information from Tables 6 and 7 can be merged in one table.
13. Line 334. “However, its bandgap can be modified by doping with other metal oxide semiconductors,..” Do you mean nanocomposites instead of “cation doping”? Explain.
14. line 423: Specify the values of the particle size.
15. line 512: see comment n. 5.
16. Reference 25: the year of publication is missing.
17. Rewrite the conclusion part
Author Response
Referee #3
Comments and Suggestions for Authors
The paper gives an overview of nanostructured metal oxides for water remediation. The subject of the paper is interesting, but the paper is sloppy written. The manuscript gives a lot of space to the research done by the Authors, instead more indications on emerging trends or recent developments should be presented, which will be of significant general interest to the journal's wide international readership. Therefore, I recommend major revisions.
General comments:
Include more points related about the difference between catalysts prepared by sol-gel and solid-state methods.
Answer: As referee requested a paragraph commenting the difference between catalysts prepared by sol-gel and solid-state methods was added in line 541
The title needs to be more focused, for example by including a reference to the preparation method via solid state.
Answer: As referee requested the title was changed.
Main comments:
- line 50: “Among the various physical, chemical, and biological technologies used in pollution control, advanced oxidation processes such as photocatalysis are being increasingly adopted in the destruction of organic contaminants due to their high efficiency, simplicity, good reproducibility, and ease of handling [8]”. For large scale municipal applications, current photocatalytic water treatment systems are less attractive because they are more time consuming and have higher costs than other existing advanced oxidation techniques such as UV/H2O2, O3/H2O2, and UV/O3technologies (see for example: Von Gunten, U. Oxidation Processes in Water Treatment: Are We on Track? Environ. Sci. Technol. 2018, 52, 5062–5075, doi:10.1021/acs.est.8b00586. Cates, E.L. Photocatalytic Water Treatment: So Where Are We Going with This? Environ. Sci. Technol. 2017, 51, 757–758, doi:10.1021/acs.est.6b06035).
Answer: As referee requested the reference was included, [9] and a sentence was incorporated in a modified paragraph line 49-56.
The Authors should include recently reported applications to prove that photocatalysis is being increasingly adopted.
Answer two recent articles showing that. photocatalysis is being increasingly adopted have been added, (86, 87) and a sentence comment on this has been added on page 21, as referee required.
- line 68: The generation of “green hydrogen” is not an environmental remediation process.
Answers The phrase “green hydrogen was removed” as referee required
- Check fig. 1: which is the meaning of the arrow with “photocatalytic study”?
Answer the arrow with “photocatalytic study” indicates how the general process represented on the left of the drawing would be applied specifically to metal oxides MxOy
- line 100: “… release the hv energy…”? This sentence is unclear.
- line 110: “… energy exceeding the band gap…” should read “…energy that matches or exceeds the band gap energy of the semiconductor…”.
Answer: The phrase was changed according to the suggestions given in 4 and 5
- Table 1. 3rd column on the top “a,b” refer to what? For each material listed in the table 1 reference must be present.
Answer the meaning of a is at the bottom of the page: aValues from references: 17-21. b Other similar values are reported for some of the metal oxides. For each material listed in the table 1 a reference was added as reference required.
7.- Table 2. For each material listed in the table reference must be present. Moreover, in the 5th column “discoloration rate “ should be “ removal %”.
Answer: Because table 2 is an adaptation of the original table, only the general reference from which the information was extracted is cited. correction in fifth column was made according to the referee requirement.
- Equation 1: Authors must specify the meaning of all terms in the equation.
Answer: all terms of the equation were specified
- line 159: Table 4 is referred to in the text before Table 3.
Answer As referee requested the correction was made
- Table 3 For each material listed in the table reference must be present.
Answer: Because table 3 is an adaptation of the original table, only the general reference from which the information was extracted, is cited.
- Table 4 For each material listed in the table reference must be present. Moreover, for each material specify the removal time.
Answer: because table 2 is an adaptation of the original table, only the general reference from which the information was extracted, is cited. Removal time are only given in compounds 1, 10, 11 and 12 from the original table.
- Tables 6 and 7. Information from Tables 6 and 7 can be merged in one table.
Answer: The information given in tables 6 is related to bandgap for TiO2 obtained using different precursors and temperatures of TiO2, while the information given in table 7 is related to kinetic data for dye degradation, so there is not much relationship between, except they both refer to some aspects of TiO2. For this reason, we believe it is more convenient to leave them separate.
- Line 334. “However, its bandgap can be modified by doping with other metal oxide semiconductors,..” Do you mean nanocomposites instead of “cation doping”? Explain.
Answer: The sentence was changed to “However, its bandgap can be modified by doping with other metal oxide semiconductors or by formation of nanocomposites in which one of the components would be in less quantity” for better clarity.
- line 423: Specify the values of the particle size.
Answer .As the referee required the particle size was added in the sentence
- line 512: see comment n. 5.
Answer I don't understand what you want to ask
- Reference 25: the year of publication is missing.
Answer. The year of publication was added as referee required
- Rewrite the conclusion part
Answer The conclusion part was rewrite as referee required
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The authors corrected the work as suggested by the reviewer. The manuscript has been sufficiently improved and can now be published in its current form.
Author Response
Thank you very much for accepting the corrections.
Reviewer 2 Report
After closely inspecting the revised MS and the detailed responses to the reviewer, I have noticed that most of problems concerted by the reviewershave been satisfactorily addressed. Thus, I recommended this article publication in its present form.
Author Response
Thank you very much for accepting the corrections
Reviewer 3 Report
The Authors have revised the manuscript , however the conclusions part is still not well organized and sloppy written.
Please rewrite the conclusions part.
Author Response
Answer:
Conclusions have been rewritten as suggested by the reviewer, left in the text with a different color. Hoping the reviewer agrees.
rewritten
Nanostructured metal oxides play an important role in the environmental decontamination of organic dyes. An alternative that often improves photocatalytic efficiency is the use of metal oxides obtained by a solid-state method. Specifically, the solid-state method based on the thermal treatment of Chitosan●MXn and PS-co-4-PVP● MXn precursors affords pure phase nanostructured metal oxides MxOy, which generally are efficient photocatalysts and often more efficient than the respective MxOy obtained by other solution methods. It was found that the photocatalytic efficiency depends on the nature of the MXn salts of the precursors and the temperature of the thermal t
reatment, as in the case of TiO2, where the most efficient photocatalyst was that obtained from the Chitosan·TiSO4 precursor pyrolyzed at 800 °C. The efficiency of most of the studied photocatalysts can be described by the following relationship:
PE = a band gap + b particle morphology + c particle size + d crystalline phase + e pyrolysis temperature
where PE is photocatalyst efficiency. Then, for a nanostructured metal oxide semiconductor photocatalyst obtained using our solid-state method, morphology can be more important than the bangap. In turn, when the preparation method of the metal oxide photocatalyst is in solid state, the temperature often modulates the morphology. This can, for example, generate a highly porous morphology, thus causing a very efficient photocatalytic activity.
However, research on this topic is a challenge since the parameters that induce a more efficient photocatalysis in solid-state synthesis are not fully known. An achievement in this matter could be a significant advance for environmental decontamination.
