TiO₂-Based Photocatalytic Coatings on Glass Substrates for Environmental Applications

Round 1

Reviewer 1 Report

This paper reviews the various deposition techniques for TiO₂-based photocatalysts. Among the long list of review papers dedicated to TiO₂ as a photocatalyst, this paper stands out for the glass substrate on which this photocatalyst is deposited as a thin film.

This is an interesting study that lists the most recent publications in the field. However, a number of shortcomings need to be addressed before it can be considered for publication.

1- First of all, it is important that the authors point out in their introduction the risks to human health associated with the use of TiO₂, which has led to its ban in all food applications and the consequent depollution of water.

https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32022R0063

2- The use of ordinary glass as a substrate offers a number of advantages, including low cost. However, it also poses a number of problems, including impurities such as sodium, which can affect the optoelectronic properties of TiO₂. Unless quartz is used, ordinary glass also blocks much of the UV spectrum, making it impossible to illuminate the photocatalyst through the substrate in a depollution reactor, for example.

3- In Table 1, the authors compare the advantages and disadvantages of different deposition techniques. This table should be consistent with the purpose of the article and limited to deposition on a glass substrate. Thus, the disadvantage of the expensive substrate for the PVD technique must be eliminated, as well as the risk of substrate degradation (due to the high temperature) or the advantage of being able to deposit on different substrates for the CVD technique.

Other comments on Table 1:

- Regardless of the deposition technique, thickness control is usually achieved by controlling the deposition time. This is not a particular disadvantage of wet chemical deposition techniques.

- Electroplating also has two major drawbacks: the need for a conductive substrate and the inability to deposit above 80°C.

- All low-temperature deposition techniques result in low material density and poor adhesion to the substrate, requiring post-deposition thermal annealing to improve material crystallinity.

In their conclusion, the authors must temper their remarks when they claim that coupling different deposition techniques to take advantage of each technique would be a solution for improving photocatalytic deposition performance. Unfortunately, the use of combined methods is often incompatible with industrial processes. For the time being, exploiting the plasmonic properties of metal nanoparticles and metal-semiconductor junction effects remains the best prospect for improving the visible absorption of large-gap semiconductors and the lifetime of charge carriers.

Author Response

To Reviewer #1

This paper reviews the various deposition techniques for TiO₂-based photocatalysts. Among the long list of review papers dedicated to TiO₂ as a photocatalyst, this paper stands out for the glass substrate on which this photocatalyst is deposited as a thin film.

This is an interesting study that lists the most recent publications in the field. However, a number of shortcomings need to be addressed before it can be considered for publication.

Response: We are very grateful for giving us a positive suggestion about our review, and we recorded it carefully and made some modifications.

Comment 1: First of all, it is important that the authors point out in their introduction the risks to human health associated with the use of TiO₂, which has led to its ban in all food applications and the consequent depollution of water.

Response: Thanks for your suggestion. According to this comment, we have added the risks to human health and the environment associated with the use of TiO₂, which makes the content more complete.

The details are shown as follows:

(Page 1, Lines 40-42)

It is worth noting that TiO₂ nanoparticles (NPs) will inevitably remain in the environment and migrate in water and soil, which poses a risk to human and environmental health.

Comment 2: The use of ordinary glass as a substrate offers a number of advantages, including low cost. However, it also poses a number of problems, including impurities such as sodium, which can affect the optoelectronic properties of TiO₂. Unless quartz is used, ordinary glass also blocks much of the UV spectrum, making it impossible to illuminate the photocatalyst through the substrate in a depollution reactor, for example.

Response: We are very grateful for your valuable question and have considered it carefully. At first, as the reviewer said:“ it also poses a number of problems, including impurities such as sodium, which can affect the optoelectronic properties of TiO₂”. In fact, if the photocatalytic coating is too thin, the sodium diffusion from the glass substrate could adversely affect the photocatalytic activity. However, this problem could be solved by increasing the thickness of the coatings, thus avoiding the possibility of sodium diffusion from the glass substrates.

Secondly, the problem is that ordinary glass blocks much of the UV spectrum. In general, the light is directly irradiated on the coated glass surface as shown in the following figure, so the problem of glass transmittance does not need to be considered. In addition, the transmittances of the FTO and ITO glasses used are 87% and 96%, respectively. The transmittance of glass will also be considered in the study of ordinary glass. For example, in “Functional photocatalytically active and scratch resistant antireflective coating based on TiO₂ and SiO₂”, the authors used the antireflection coatings, which in the designed wavelength range had transmittance higher than 97%.

Comment 3: Comments on Table 1.

Response: We appreciate the instructive comment raised by the reviewer. According to the reviewer's comments, we rewrote some parts of the Table 1.

Comment 4: In their conclusion, the authors must temper their remarks when they claim that coupling different deposition techniques to take advantage of each technique would be a solution for improving photocatalytic deposition performance. Unfortunately, the use of combined methods is often incompatible with industrial processes. For the time being, exploiting the plasmonic properties of metal nanoparticles and metal-semiconductor junction effects remains the best prospect for improving the visible absorption of large-gap semiconductors and the lifetime of charge carriers.

Response: We sincerely appreciate the reviewer’s suggestion. We have rewritten this part of the conclusion according to the reviewer’s comment. We want to thank the reviewer for his comments and suggestions. These constructive comments are critical and helpful for further improving our manuscript.

The details are shown as follows:

(Page 21, Lines 634-639)

Coupling different deposition techniques to take advantage of each technique would be a solution for improving photocatalytic deposition performance. Unfortunately, the use of combined methods is often incompatible with industrial processes. At present, it is still the best prospect to improve the photocatalytic performance of TiO₂-based coatings by using the plasma characteristics and metal-semiconductor junction effect of metal NPs.

Reviewer 2 Report

I have thoroughly read the entitled MS and found it interesting and important to the scientific community. The MS demonstrates the importance of TiO₂-based materials and discusses their coating techniques. I recommend accepting the MS after the following revisions:

-          The English of the MS must be edited. Some examples:

Line 98: add ‘and’ before latex.

Lines 100-101: This sentence is not correct in English. Glass with capital G. ‘fabricated’ instead of ‘fabrication’, ‘stable’ instead of ‘stability’, etc.

Line 109: ‘and’ before chemical.

Line 124: Delete ‘which could be used to control the thickness’ as it is repeated in the same sentence.

Lines 132-133: The sentence ‘The number of coating cycles could increase as needed for dip-coatings, while the increased dip-coating cycles only increase the coating’s thickness without the crystal phases changing’ repeats itself.

Line 145: the temperature range 400 oC and 600 oC was already mentioned.

Line 147: replace ‘annealed’ with ‘annealing.’

Line 544: the word sors should be spelled correctly.

-          Line 72 – The authors claim that ‘brookite TiO₂ has relatively low photocatalytic activity.’  The activity order of the TiO₂ phases does probably not depend only on the phase as some contradicting results were reported. See https://doi.org/10.1039/B612172A where in their experiment, Rutile is the worst and brookite is the best, and https://doi.org/10.1016/j.materresbull.2020.110842 showing that brookite-anatase mixed phase is better than pure anatase.

-          Line 136 – Replace ‘atoms’ with ‘ions.’

-          Figures 5d and 5f are nor readable.

-          Line 172 – Define ‘RMS’.

-          Section 3.2 - The electrodeposition should be better explained - how does the electric potential assist the coating?

-          Line 344 – Define ‘PEC.’

-          Line 381 – Delete the word ‘atmosphere.’

-          Line 483 – The reader may not understand the source of Cl-.

-          Line 525 – replace ‘disorder’ with ‘disordered.’

-          Line 530 – The word 'however' does not fit here. You can use 'additionally.'

-          Line 549 – what does 16 in 16TiCuO stand for?

-          Line 569 – I see four techniques in the table, not three.

-          Line 571 – It would be nice to tell the reader which method you meant.

-          Line 582 – what was the precursor for ZnIn2S₄?

-          Line 587 – What is the band gap 3.2 eV as written here or 3.6 eV as in figure says?

See my comments in 'Comments and Suggestions for Authors'.

Author Response

I have thoroughly read the entitled MS and found it interesting and important to the scientific community. The MS demonstrates the importance of TiO₂-based materials and discusses their coating techniques. I recommend accepting the MS after the following revisions:

The English of the MS must be edited. Some examples:

Line 98……Line 544; Line 136……Line 172; Line 344……Line 571

Response: We appreciate the instructive comment raised by the reviewer. We feel sorry about these mistakes. In order to accurately introduce the research content of the manuscript, we carefully corrected the language and writing problems and marked the significant changes in the compared file.

Comment 1: Line 72 – The authors claim that ‘brookite TiO₂ has relatively low photocatalytic activity.’  The activity order of the TiO₂ phases does probably not depend only on the phase as some contradicting results were reported.

Response: We sincerely appreciate the reviewer’s suggestion. We have rewritten this part according to the reviewer’s comment.

The details are shown as follows:

(Page 2, Lines 75-78)

All of these crystalline structures could act as photocatalysts. The effect of the crystal phase on the photocatalytic performance of TiO₂ has attracted extensive attention. At present, pure- and mixed-phase TiO₂ photocatalysts have been synthesized and applied in photocatalysis.

Comment 2: Section 3.2 - The electrodeposition should be better explained - how does the electric potential assist the coating?

Response: Thanks a lot for your comment. We fully agree with the reviewer’s comment. We have supplemented the content of the electric potential during the electrodeposition process according to the reviewer’s comment, which enriches the content of this part of the manuscript.

Comment 3: Line 582 – what was the precursor for ZnIn₂S₄?

Response: We are very grateful for your valuable comment. We have added the content of the precursor for ZnIn₂S₄.

The details are shown as follows:

(Page 19, Lines 601)

The nanocrystalline Znln₂S₄ films were deposited on the ITO-coated glass substrate using ZnCl₂, InCl₃, and Na₂S₂O₃ž5H₂O as the precursors by electrodeposition at room temperature.

Comment 4: Line 587 – What is the band gap 3.2 eV as written here or 3.6 eV as in figure says?

Response: Thanks for your suggestions. We feel sorry about this mistake. The content should be consistent with the value in the picture, both of which are 3.6 V.

Reviewer 3 Report

This work presents the research progress of heterogeneous TiO2-based photocatalytic coatings deposited on glass substrates using various deposition techniques. These TiO2-based composite coatings, obtained through different methods, have demonstrated excellent performance in self-cleaning, pollution removal, air purification, and antibiosis. The study discusses the different deposition techniques employed to prepare TiO2 coatings and highlights their photocatalytic applications with references to relevant literature.

The review manuscript provides essential information on various deposition techniques and SEM, EDS, UV-Vis, and FTIR analysis results. These details will be valuable to any reader interested in delving into the various TiO2 thin film deposition methods. In conclusion, I recommend publishing the manuscript in its current format.

Minor editing of English language required

Author Response

Thank you for your encouraging comment! In the future, we will continue to do our best on the road of scientific research.

Reviewer 4 Report

The manuscript "TiO2-based photocatalytic coatings on glass substrates for environmental applications" reviews the literature about methods for forming TiO2 coatings for environmental applications.  The topic is relevant to the scientific community and the paper is well organized, especially for researchers that are looking for a well-written paper which describes the leading technologies for such applications. Although its clarity some small changes are required before the acceptance of the paper.

GENERAL

I suggested to add at the end of the papers a list of the abbreviations used in the manuscript.

INTRODUCTION

Line 41-42: I suggest to change "this problem" with the plural form being the issues presented in the previous part of the text more than one.

2. THE BASIC PRINCIPLE...
Line 68-69: "It is shown in Figure 2..." I suggested to rewrite this sentence, because in the current form seems or incomplete or not so clear.
Line 69-70: the suggestion is to discuss in the text the different TiO2 phase in the same order of Figure 2, then anatase, rutile and brookite

Line 80: rewrite the sentence "IT is illustrated in Figure 3..." at the moment is really hard to follow

Finally, I suggested implementing a little bit more of the current challenges that in this version are limited to the conclusions.

The paper is generally well-written. Some sentences require a reformulation to improve paper readability.

Author Response

The manuscript "TiO₂-based photocatalytic coatings on glass substrates for environmental applications" reviews the literature about methods for forming TiO₂ coatings for environmental applications.  The topic is relevant to the scientific community and the paper is well organized, especially for researchers that are looking for a well-written paper which describes the leading technologies for such applications. Although its clarity some small changes are required before the acceptance of the paper.

Response: We are very grateful for giving us a positive suggestion on this review, and we recorded it carefully and made some modifications.

Comment 1: I suggested to add at the end of the papers a list of the abbreviations used in the manuscript.

Response: Thanks for your suggestion. According to the reviewer's suggestion, we have added a list of abbreviations at the end of the paper.

The details are shown as follows:

(Page 21, Lines 652-667)

Abbreviations:

TiO₂, Titanium dioxide; WCD, Wet chemical deposition; PVD, Physical vapor deposition; CVD, Chemical vapor deposition; AOPs, Advanced oxidation process; NPs, Nanoparticles; Eg, Bandgap; VB, Valence band; CB, Conduction band; e⁻, Photogenerated electron; h⁺, Positivity-charged hole; O₂^•−, Superoxide anion radical; •OH, Hydroxyl radical; •OOH, hydroperoxide radical; BB-41, Basic Blue 41; MB, Methylene blue; ZOC, Zirconium (IV) oxychloride octahydrate; TTIP, Titanium (IV) isopropoxide; RMS, Root mean square; FTO, Fluorine-doped tin oxide; RB 19, reactive blue 19; Au, Gold; SOG, Spin-on-glass; EY, Eosin yellow; EP, Epoxy resin; F-PMHS, Fluorine-containing hydrogen-containing polysiloxane; ITO, Indium-doped tin oxide; PEC, Photoelectrocatalytic; DC, Direct current; RF, Radio frequency; SA, Stearic acid; PLD, Pulsed laser deposition; Cr(VI), Dichromate; EO, Electro-oxidation; AA-CVD, Aerosol-assisted CVD; AP-CVD, Atmospheric pressure CVD; PE-CVD, Plasma-enhanced CVD; AA-MO-CVD, Aerosol-assisted metal-organic CVD; B, Boron; B[(CH₃)₂CHO]₃, Boron isopropoxide; TiCl₄, Titanium tetrachloride; (EtO)₃PO, Triethyl phosphate; sccm, standard cubic centimeters per minute.

Comment 2: Line 68-69: "It is shown in Figure 2..." I suggested to rewrite this sentence, because in the current form seems or incomplete or not so clear.
Line 69-70: the suggestion is to discuss in the text the different TiO₂ phase in the same order of Figure 2, then anatase, rutile and brookite.
Response: We are very grateful for your helpful suggestions and have rewritten this part carefully.

The details are shown as follows:

(Page 2, Lines 71-75)

In all these three forms, Ti atoms are coordinated to O atoms, forming TiO₆²⁻ octahedral units. The formation of cells for three TiO₂ depends on how TiO₆²⁻ octahedral units contact with the neighboring octahedrons. It is shown in Figure 2 that adjacent octahedra units in anatase, rutile, and brookite share four, two, and three edges, respectively. All of these crystalline structures could act as photocatalysts.

Comment 3: Line 80: rewrite the sentence "IT is illustrated in Figure 3..." at the moment is really hard to follow.

Response: Thanks for giving us valuable advice. We have rewritten this sentence according to the reviewer’s comment.

The details are shown as follows:

(Page 3, Lines 82-83)

The photocatalytic process for TiO₂ involving the formation of the advanced oxidants is illustrated in Figure 3.

Comment 4: Finally, I suggested implementing a little bit more of the current challenges that in this version are limited to the conclusions.

Response: We appreciate the reviewer’s valuable suggestion of our work. According to the reviewer's comments, we have appropriately supplemented the content of the current challenge in Section 3.

The details are shown as follows:

(Page 5, Lines 175-179)

The obtained mesoporous TiO₂-ZrO₂ coated float glass had self-cleaning capability, photocatalytic performance, and anti-reflective properties (Figure 5d), which could effectively replace the empty texture solar cover glass in the market. However, the performances of the TiO₂-ZrO₂ coated float glass on a large scale needed to be evaluated in actual applications.

(Page 6, Lines 210-215)

Compared with commercial Pilking-tonActiv^TM, embedding Au NPs leads to better photocatalytic self-cleaning activity under dark and irradiation conditions. The plasmonic effect of Au NPs was fully utilized to alleviate the problem of poor visible light response of TiO₂ coating. Therefore, the self-cleaning glass with Au-TiO₂ composite coatings had great application potential under actual sunlight. Further study is needed to verify the self-cleaning application of the developed photocatalytic coatings in real life.

(Page 9, Line 316 – Page 10, Line 320)

Therefore, these excellent properties allowed M-TiO₂@EP coating could be a candidate for promising outdoor and harsh condition applications. Unlike the previous preparation steps, the composite coating was prepared by one-step spraying. The preparation method is simple, which makes the coating possible for large-scale applications.

(Page 13, Lines 441-444)

In addition, although the N-doping led to a narrower band gap, it caused faster recombination of the active photoproduction species. Therefore, N doping did not cause an improvement in photocatalytic activity.

(Page 17, Lines 541-546)

P groups (P⁵⁺ species and P³⁻ species) affect the photocatalytic activity differently. In the P-TiO₂ composite coatings containing only P⁵⁺ species, charge carrier concentrations increased by several orders of magnitude, while incorporating P³⁻ species seriously reduced the photogenerated carrier lifetimes. Additionally, the photocatalytic performance of the P ions-TiO₂ composite coatings was also reduced, which was proved in the SA mineralization results.

(Page 18, Lines 565-568)

TiCuO composite coatings were promising, non-toxic fouling release films for marine and industrial applications. However, an effective antifouling surface in practical applications should resist various fouling trophic scales, which requires further research.

Round 2

Reviewer 1 Report

The authors have incorporated the comments I made in my report.