Composites for Aqueous-Mediated Heterogeneously Catalyzed Degradation and Mineralization of Water Pollutants on TiO2—A Review
Round 1
Reviewer 1 Report
Overall the review is well-conceived and the manuscript well-written. The authors have done an extensive literature review (196 positions). The article raises many issues, making it more interesting.
1) I would improve the quality of the Figures 1, 6a and 8 - too small letters making the description poorly visible,
2) Additionally, when authors wrote for example : „Different sorts of color dyes are accessible in the industry sectors as dyeing/shading objects. „ – page 8 , line 284. Or „Pharmaceuticals include a broad category of human and veterinary medicines that have been continuously used in huge quantities around the world.” - page 8, line 327. I would list 2- 3 examples each in parentheses.
In my opinion, after these minor adjustments, it can be published.
Author Response
We thank the reviewers for recommending and appreciating our work for consideration after revision. We are pleased to consider all the suggestions provided by them for improving the quality of the manuscript during revision. The answers to all the questions as listed below.
Reviewer #1
Comments and Suggestions for Authors.
Overall, the review is well-conceived and the manuscript well-written. The authors have done an extensive literature review (196 positions). The article raises many issues, making it more interesting. In my opinion, after these minor adjustments, it can be published.
1) I would improve the quality of the Figures 1, 6a and 8 - too small letters making the description poorly visible,
Response: As per the reviewer’s suggestion, the quality of the above-mentioned figures is updated and attached in the revised manuscript.
2) Additionally, when authors wrote for example: Different sorts of color dyes are accessible in the industry sectors as dyeing/shading objects. „ – page 8, line 284. Or „Pharmaceuticals include a broad category of human and veterinary medicines that have been continuously used in huge quantities around the world.” - page 8, line 327. I would list 2- 3 examples each in parentheses.
Response: As per the reviewer’s suggestion, the examples for dye and pharmaceutical pollutants were added in the parentheses in the revised manuscript.
Reviewer 2 Report
Dear author,
Complements on your efforts. The study is well structured and detailed.
Here are some of my recommendation to make it even better.
1) Revise some of the phrasing in your text, some of the expressions are not appropriately formulated and sound unsuitable.
2) It would be appropriate to mention and make comparison, albeit briefly, to ZnO, the other widely explored heterogeneous photocatalytic oxide material.
3) The titles of section 4.3 (row 521) and 4.4 (row 579) coincide.
4) It would be relevant to include another very significant class of organic pollutants in water - hydrocarbons/petroleum derivatives. There are works based on photocatalytic decomposition of such pollutants y TiO2 and ZnO.
Author Response
We thank the reviewers for recommending and appreciating our work for consideration after revision. We are pleased to consider all the suggestions provided by them for improving the quality of the manuscript during revision. The answers to all questions are given in the attached file.
Author Response File: 
Author Response.docx
Reviewer 3 Report
Writing a review is time-consuming thing, and the approach should be broader than the authors present. Reviews play the main role in research as they determine the direction of investigations.
There are some remarks to the presented text of the manuscript.
1. Section heading 2 missing.
2. Authors present a review on titanium dioxide as a heterogeneous catalyst. Hence, one should consider not only the anatase phase, which is guessed from the value of the band gap, but also other phases (rutile, brookite).
3.What does χ mean in formula (1)?
4. According to Fig. 4, O2- radicals are also involved in reactions with pollution. But in the reaction scheme (3)-(6) this is not noted.
5. Page 6 lines 214-215: The sentence is incomprehensible. It assumes that the value of the bandgap will be another under visible light.
Page 6, lines 232-234: The assertion is incorrect. In terms of bandgap values, the use of rutile is preferable.
Page 6 lines 239-241: What are the advantages of an indirect transition over a direct transition?
6. The title of subheading 3.1 does not match the content.
7. Page 7 lines 251-253: Incorrect sentence. According to reference 62, the Fe2+, Fe3+, S2O82- ions, and H2O2 are added to the photocatalytic system (into liquid phase). The addition to the photocatalytic structure is doping, which is very difficult to carry out in the case of hydrogen peroxide.
8. Page 8 line 288: reference 64 does not match the text. Link 65 is more appropriate.
9. page 9: Reactions (16)-(18) are reactions (3)-(5).
10. page 10: It is known that the zero point charge changes with the synthesis temperature and, as a consequence, depends on the anatase : rutile phase ratio (Sanchez, L., Peral, J., & Domenech, X. (1998). Aniline degradation by combined photocatalysis and ozonation. Applied Catalysis B: Environmental, 19(1), 59-65.). Also, the doping of TiO2 leads to the appearance of an additional charge on the surface. Examples are given for doped titanium dioxide or composites based on it. What are the pHzpc values of these catalysts?
11. Subsection 4.2: A simple analysis shows that in any advanced oxidation technology system, the higher the contaminant concentration, the lower the rate of removal/decomposition. And what is the reason for this? The review needs to draw some conclusions.
12. Subswction 4.3: Yes of course, everyone agrees that the more catalyst is loaded into the system, the greater the degree of removal. But if some authors note the presence of a saturation point (the critical mass of the catalyst), then the opinion of the authors of the review on this issue is more interesting. What is the reason for this?
13. The Titles of subsections of 4.3 and 4.4 are the same.
14. page 14 lines 592-592: H2O2 decomposes under the light, and reactions (22) and (23) can proceed only theoretically, since electrons and a hole move towards the catalyst surface under the action of light. The presence of hydrogen peroxide in the system is an additional oxidizing agent interacting directly with the pollutant.
15. page 16 Incorrect examples: in [85] the same time is given but different classes of dyes; [195] one value of the luminous flux is presented; [176] used a composite in which the value of the band gap was reduced.
16. The text of the manuscript does not provide examples of doped titanium dioxide, which is actively used in photocatalysis processes by irradiated with visible light.
Author Response
We thank the reviewers for recommending and appreciating our work for consideration after revision. We are pleased to consider all the suggestions provided by them for improving the quality of the manuscript during revision. The answers to all the questions as listed below.
Reviewer #3
Writing a review is time-consuming thing, and the approach should be broader than the authors present. Reviews play the main role in research as they determine the direction of investigations.
There are some remarks to the presented text of the manuscript.
- Section heading 2 missing.
Response: As per the reviewer’s suggestion, all the section headings are added to the revised manuscript.
- Authors present a review on titanium dioxide as a heterogeneous catalyst. Hence, one should consider not only the anatase phase, which is guessed from the value of the band gap, but also other phases (rutile, brookite).
Response: Herein, we are not proving one phase of TiO2 shows better optical activity over others. But we are focusing on how phases of TiO2 make an impact on light absorption, recombination of charges, and catalytic activity.
3.What does χ mean in formula (1)?
Response: χ(AaBbCc) represents the absolute electronegativity of semiconductor materials dictated by 'Pearson’s Absolute Electronegativity' with the outright electronegativities of metal particles and anions.
- According to Fig. 4, O2- radicals are also involved in reactions with pollution. But in the reaction scheme (3)-(6) this is not noted.
Response: Yes, the O2●― radicals are also involved in reactions with pollution and as per the reviewer’s suggestion, reaction scheme (6) was modified and added in the revised manuscript.
- Page 6 lines 214-215: The sentence is incomprehensible. It assumes that the value of the bandgap will be another under visible light.
Response: As per the reviewer’s suggestion, the above-mentioned paragraph is modified and added in the revised manuscript.
Owing to the wide bandgap (3.2 eV) under UV-visible light, TiO2 has been evidenced to be among the favorable n-type semiconductor. This material is also effectively improved because of its excellent chemical as well as physical stability, and its greater refractive index. Several novel TiO2 photocatalysts have indeed been reported, with which most of that UV- illuminated photoreactions are catalyzed, and nowadays TiO2 is modified to sensitize the visible light potentially.
Page 6, lines 232-234: The assertion is incorrect. In terms of bandgap values, the use of rutile is preferable.
Response: Thank you for pointing out the mistake and it is corrected in the revised manuscript.
Herein, we are not proving one phase of TiO2 shows better optical activity than others. But we are focusing on how phases of TiO2 make an impact on light absorption and catalytic activity.
Page 6 lines 239-241: What are the advantages of an indirect transition over a direct transition?
Response: The exact reverse of radiative recombination is light absorption. For the same reason as above, light with photon energy close to the band gap can penetrate much farther before being absorbed in an indirect band gap material than a direct band gap one. The indirect process proceeds at a much slower rate, as it requires three entities to intersect in order to proceed: an electron, a photon, and a phonon. This is analogous to chemical reactions, where, in a particular reaction step, a reaction between two molecules will proceed at a much greater rate than a process that involves three molecules
- The title of subheading 3.1 does not match the content.
Response: As per the reviewer’s suggestion, section subheadings 3.1. is modified and added in the revised manuscript.
3.1. Influence of Distinct Phase, Band gap energies of TiO2 semiconductor and reactive species over photocatalysis
- Page 7 lines 251-253: Incorrect sentence. According to reference 62, the Fe2+, Fe3+, S2O82- ions, and H2O2 are added to the photocatalytic system (into liquid phase). The addition to the photocatalytic structure is doping, which is very difficult to carry out in the case of hydrogen peroxide.
Response: As per the reviewer’s suggestion, the suggested changes have been carried out in the revised manuscript.
The band tuning can be acknowledged additionally by doping the semiconductor. To overcome this trouble, a few added substances such as Fe2+, Fe3+, metal and non-metals doping, and so forth, have been added to the photocatalytic structure.
- Page 8 line 288: reference 64 does not match the text. Link 65 is more appropriate.
Response: As per the reviewer’s suggestion, the suggested changes have been carried out in the revised manuscript.
Reference 64 [before modification] is removed and we changed reference 65 [before modification] to reference 64.
- page 9: Reactions (16)-(18) are reactions (3)-(5).
Response: As per the reviewer’s suggestion, reaction numbers (16)-(18) are removed in the revised manuscript.
Reactions (16)-(18) are reactions (3)-(5) both are same only.
- page 10: It is known that the zero-point charge changes with the synthesis temperature and, as a consequence, depends on the anatase: rutile phase ratio (Sanchez, L., Peral, J., & Domenech, X. (1998). Aniline degradation by combined photocatalysis and ozonation. Applied Catalysis B: Environmental, 19(1), 59-65.). Also, the doping of TiO2 leads to the appearance of an additional charge on the surface. Examples are given for doped titanium dioxide or composites based on it. What are the pHzpc values of these catalysts?
Response: Some of the pHzpc values are given in the manuscript.
Zero-point charge (pHzpc) exists at 3.5 for the undoped and 3.84 for CuSO4-doped TiO2 were resolved. Also, the assistance of TiO2 found the zero-point charges (pHzpc) at pH = 6.25.
- Subsection 4.2: A simple analysis shows that in any advanced oxidation technology system, the higher the contaminant concentration, the lower the rate of removal/decomposition. And what is the reason for this? The review needs to draw some conclusions.
Response: It would be accounted for by expanding the substrate molecule concentration, and the kinetics rate of mineralization reaction (k) diminishes. The underlying substrate pollutant concentration can promote photocatalytic efficacy dependent on two fundamental viewpoints. In exalted concentrations of toxic pollutants, more reactant sites are observed on the TiO2 surface by pollutant substrates substance, which diminishes the hydroxyl (OH●) radical origination, since there are fewer active sites for sticking to anions like hydroxyl ions. Moreover, the foreign substances consumed the photons prior to arriving at the photocatalyst surface, subsequently, diminishing the photocatalytic proficiency.
- Subsection 4.3: Yes of course, everyone agrees that the more catalyst is loaded into the system, the greater the degree of removal. But if some authors note the presence of a saturation point (the critical mass of the catalyst), then the opinion of the authors of the review on this issue is more interesting. What is the reason for this?
Response: This review is basically focussing on the type of photocatalyst and their dosage. To resist the abuse of photocatalysts, it is predominant to ascertain the loading or optimized dose of the catalyst sample for the proficient eradication of toxic organic and dye pollutants. The amount of photocatalyst required will rely upon the photocatalytic mechanism, light source, toxic pollutants concentration, and the sort just as molecule morphology of the photocatalyst. Reduced photocatalyst amounts increase suspension transparency, which extends the perforate of photon illumination in the photoreactor and so reduces the photocatalytic degradation rate. Thus, there may be a saturation point beyond which the catalyst dose has no direct relationship to the percentage of photodegradation.
- The Titles of subsections of 4.3 and 4.4 are the same.
Response: The titles of sub-sections 4.3 and 4.4 are different, not the same.
4.3. Influence of photocatalyst loading into reaction suspension:
4.4. Performance of Hydrogen peroxide (H2O2) oxidizing species
- page 14 lines 592-592: H2O2 decomposes under the light, and reactions (22) and (23) can proceed only theoretically, since electrons and a hole move towards the catalyst surface under the action of light. The presence of hydrogen peroxide in the system is an additional oxidizing agent interacting directly with the pollutant.
Response: As per the reviewer’s suggestion, the suggested changes have been carried out in the revised manuscript.
The initial pollutant substrate concentration is the essential requirement for the photocatalytic approach, and substrate samples are adsorbed over the reactive surface of the utilized catalyst. Photocatalysis depends on the adsorption of pollutants on the surface of the photocatalyst. In the photocatalysis process, only the amount of pollutants adsorbed on the surface of the photocatalyst contributes and not the one in the bulk of the solution. The adsorption of pollutants depends on the initial concentration of the dye. The initial concentration of pollutants in a given photocatalytic reaction is an important factor that needs to be considered. It would be accounted for by expanding the substrate molecule concentration, and the kinetics rate of mineralization reaction (k) diminishes. The underlying substrate pollutant concentration can promote photocatalytic efficacy dependent on two fundamental viewpoints. In exalted concentrations of toxic pollutants, more reactant sites are observed on the TiO2 surface by pollutant substrates substance, which diminishes the hydroxyl (OH●) radical origination, since there are fewer active sites for sticking to anions like hydroxyl ions. Moreover, the foreign substances consumed the photons prior to arriving at the photocatalyst surface, subsequently, diminishing the photocatalytic proficiency.
- page 16 Incorrect examples: in [85] the same time is given but different classes of dyes; [195] one value of the luminous flux is presented; [176] used a composite in which the value of the band gap was reduced.
Response: Reference 85 [before modification] now 84: The photodegradation of three different types of dyes like an anionic dye, a cationic dye, and a zwitterionic dye, namely, Reactive Red 2 (RR), Methylene Blue (MB), and Rhodamine B (RB), respectively, have been carried out using chitosan /TiO2 composite (CTC). They show the viable activity of chitosan /TiO2 composite over different dyes in the same reaction time.
Reference 195 [before modification] now 194: Low pressure, 39.5 W UV lamps (λ- 254 nm) having a length of 0.8 m were input-photons. The UV lamps used had identical output ratings (input rating -39.5 W and output rating -13.7 W)
Reference 176 [before modification] now 175: The absorption edges of pure ST01 TiO2 and ZnFe2O4 start at 380 and 550 nm, respectively, which are consistent with the reported data of anatase TiO2 and ZnFe2O4.The UV-visible spectra of 1 wt% ZnFe2O4–TiO2 show an absorption in the visible light region, which indicates that the extension of the absorption edge to the visible light region is mainly attributed to the existence of ZnFe2O4. The decrease in photoluminescence intensity of 1 wt% ZnFe2O4–TiO2 nanocomposite indicates the obvious retardation of electrons and holes, which is in good agreement with our previous results.
- The text of the manuscript does not provide examples of doped titanium dioxide, which is actively used in photocatalysis processes by irradiated with visible light.
Response: Thank you for your suggestion. We are planning to write another review, particularly in the area of doped titanium dioxide in the near future, and hence not included in this review article.
Round 2
Reviewer 1 Report
In my opinion, the article is suitable for publication.
Author Response
Thanks for the acceptance
Reviewer 2 Report
Thank you for your consideration of my comments.
Just few minor things I noticed in the revised version of the manuscript:
1) In Fig. 1 in the section of Homogeneous catalysis - Benefits - it is written "eaction", you should correct that typo.
2) "TiO2 is extinct in three structures....." (row 233) - I think you meant so say "TiO2 is existing in three structures...."
3) "......pollutants are stubborn chemicals....." (row 400) - I would recommend the term persistent instead of stubborn.
Thank you for your time!
Author Response
Reviewer # 2
- In Fig. 1 in the section of Homogeneous catalysis - Benefits - it is written "eaction", you should correct that typo.
Response: As per the reviewer’s suggestion, the suggested changes have been carried out in the revised manuscript.
- "TiO2 is extinct in three structures....." (row 233) - I think you meant so say "TiO2 is existing in three structures...."
Response: As per the reviewer’s suggestion, the suggested changes have been carried out in the revised manuscript.
- "......pollutants are stubborn...." (row 400) - I would recommend the term persistent instead of stubborn.
Response: As per the reviewer’s suggestion, the suggested changes have been carried out in the revised manuscript.
Reviewer 3 Report
Thanks to the authors for improving the text. After editing, the text of the manuscript became more readable and corresponds to the review with critical remarks.
Author Response
Thanks for the acceptance.
Round 3
Reviewer 2 Report
Dear authors,
All the suggestions have been taken into account and I am pleased with the recent version of the manuscript.
