Highly Active under VIS Light M/TiO₂ Photocatalysts Prepared by Single-Step Synthesis

Round 1

Reviewer 1 Report

In this work, a single-step impregnation approach is investigated as a synthetic route for photocatalysts synthesis active under visible light. The as-derived photocatalysts exhibited very high degradation rates towards the methylene blue (MB) decolorization under visible light despite the high concentration of the initial MB solution concentration. This work can provide some information for us, but there are still some issues. I think this manuscript can be reconsidered for publication after a major revision. The detailed comments are as follows:

1. The language of the article needs to be reorganized. A more logical and scientific organized version should be provided to the readers.

2. The wavelength range and intensity of light need to be given.

3. Some conclusions in the manuscript need to be supported by relevant literatures. The following literature is recommended to cite: ACS Mater. Au 2021, 1, 37−54; Catalysis Communications 162 (2022) 106371; ACS Mater. Au 2021, 1, 37−54; Catalysts 2023, 13, 544.

4. Brunauer-Emmett-Teller (BET) and photoelectrochemical measurements are suggested to be supplemented.

5. The format of references should be checked with great attention to meet the requirements of a journal.

 

6. The quality of the pictures should be improved.

Author Response

Reviewer #1

In this work, a single-step impregnation approach is investigated as a synthetic route for photocatalysts synthesis active under visible light. The as-derived photocatalysts exhibited very high degradation rates towards the methylene blue (MB) decolorization under visible light despite the high concentration of the initial MB solution concentration. This work can provide some information for us, but there are still some issues. I think this manuscript can be reconsidered for publication after a major revision. The detailed comments are as follows:

1. The language of the article needs to be reorganized. A more logical and scientific organized version should be provided to the readers.

Reply: We would like to thank the reviewer for this comment. The text has been logically structured in sections that declare the content for the reader convenience. In the first part a literature review has been performed with articles relevant to our research with state of the art methods and relevant materials and physicochemical characterization of the corresponding materials.

 In the second (materials and methods) part, all the chemical compounds that were used are being  mentioned, the experimental approach for the catalysts’ synthesis is described, the characterization techniques that were employed as well as a detailed description of the self-prepared photocatalytic cell where the photocatalytic experiments took place. In the results and discussion part all the characterization data along with the photocatalytic results are demonstrated and discussed. The conclusions section summarizes all the results that were conducted from the experimental results and the relevant literature. Following this structure, the reader can be introduced to metal doped titania photocatalysts and then be familiarized  to the photocatalyst preparation method.   Then the reader is updated on the results that were found and the conclusions drawn by them. Finally, the text has undergone extensive proofreading to correct any mistakes and to provide a better version of the manuscript.

2. The wavelength range and intensity of light need to be given.

Reply: We would like to thank the reviewer for this comment that permits us to improve the details provided to the reader. The information was added to the text in line 195 in the revised manuscript “light intensity 3500lux, Ra>80, Pf>0.5, Energy class A+) with wavelength range 400-700nm”

3. Some conclusions in the manuscript need to be supported by relevant literatures. The following literature is recommended to cite: ACS Mater. Au 2021, 1, 37−54; Catalysis Communications 162 (2022) 106371; ACS Mater. Au 2021, 1, 37−54; Catalysts 2023, 13, 544.

Reply: We would like to thank the reviewer for the suggested articles that were very interesting and improved the literature review of the manuscript. All the proposed articles were added to the text (Lines 64-66 and 128-135 in the revised manuscript, references: 13, 34, 35).

“Furthermore, graphene oxide has been widely used as the precursor of graphene (GR) to synthesize graphite-based hybrid photocatalysts for solar-to-chemical energy con-version.[13]”

“Moreover, Ni(OH)2-based cocatalysts, have attracted increasing research interest in the field of solar-to-fuel conversion and in particular photocatalytic H2 production and CO2 reduction[34]. Last but not least, cobalt sulfide-based compo-sites have also exhibited promising results in the field of solar fuel conversion and are being considered good candidates in photocatalytic hydrogen production, carbon di-oxide reduction, nitrogen fixation, and photocatalytic degradation of pollutants due to their low cost and easy way of synthesis as well as their diverse structures [35].”

4. Brunauer-Emmett-Teller (BET) and photoelectrochemical measurements are suggested to be supplemented.

Reply: We would like to thank the reviewer for the suggestion. The photocatalytic activity of the prepared materials is determined not only by the metal addition, but also the specific surface area. Thus, we would like to give an insight of the available specific area of the photocatalyst which facilitates the activity of the catalyst. From our point of view, the BET and photoelectrochemical measurements draw the complete picture of our photocatalysts and thus, it is important to maintain them in the main text.

5. The format of references should be checked with great attention to meet the requirements of a journal.

Reply: We would like to thank the reviewer for this comment that permits us to improve our manuscript. All the references were carefully checked and revised to meet the journal’s format.

6. The quality of the pictures should be improved.

Reply: We would like to thank the reviewer for this comment that provides us the opportunity to better display our results. All the pictures were replaced according to the reviewer’s request.

Author Response File: Author Response.docx

Reviewer 2 Report

1.The XRD analysis in this article is only the analysis of matrix TiO2, lacking the XRD and SEM analysis of M/TiO2.

 2. Only nitrogen adsorption and desorption experiments on Cu/TiO2 samples have been conducted, and no relevant experiments on Ag/TiO2 samples have been conducted. The conclusions drawn from this are relatively one-sided.

 3. Cyclicity is also an important factor in evaluating the performance of photocatalysts. This article did not conduct photocatalytic cycling experiments.

 4. There is a lack of proof experiments on the mechanism of photocatalytic degradation of MB, and the inferred conclusions need to be proven.

Author Response

Reviewer #2

The XRD analysis in this article is only the analysis of matrix TiO2, lacking the XRD and SEM analysis of M/TiO2.

Reply: We would like to thank the reviewer for this comment. Please let us further explain our thinking. The as-prepared M/TiO2 photocatalysts were analyzed by XRD but the metallic elements were not detected by the instrument due to their low concentration that did not exceed the detection limit of the instrument. Thus, XRF analysis was considered to be more suitable characterization technique for our samples. Regarding the SEM analysis, at this point, we are more interested in the photocatalyst synthesis steps optimization regarding its application and not to proceed with fundamental structural analysis of the prepared catalysts.

2. Only nitrogen adsorption and desorption experiments on Cu/TiO2 samples have been conducted, and no relevant experiments on Ag/TiO2 samples have been conducted. The conclusions drawn from this are relatively one-sided.

Reply: We would like to thank the reviewer for this comment. Please let us further elaborate on this matter. Nitrogen adsorption-desorption experiments were performed for pure anatase and rutile substrates as well as for Cu/TiO2 samples with various loadings. The adsorption/desorption curves of Cu/TiO2 was similar to the corresponding ones of the substrates. Thus, we assume, that  that the samples prepared with the same method, using the same substrates, and the same low 0.1-0.5% would not result in insignificant variations in the adsorption and desorption curves.

3. Cyclicity is also an important factor in evaluating the performance of photocatalysts. This article did not conduct photocatalytic cycling experiments.

Reply: We would like to thank the reviewer for the observation. We agree with the reviewer that cycling experiments can provide important data. In this manuscript though, we aimed to provide an initial investigation of our photocatalysts depending on their preparation steps and reagents. This is ongoing experimental research, and we aim to perform these tests in our future work.

4. There is a lack of proof experiments on the mechanism of photocatalytic degradation of MB, and the inferred conclusions need to be proven.

Reply: We would like to thank the reviewer for this comment. From our point of view the mechanism of photocatalytic degradation of MB is well established and analyzed in many scientific papers. The mechanism of the reaction that is being performed has been extensively studied in the literature with various of characterization methods to verify each and every step of the mechanism. Relevant study has been added as reference in the manuscript [5] .

Nevertheless, this is not the aim of the current study to investigate the mechanism but to validate the particular photocatalyst performance on the reaction of MB degradation.

Our conclusions are based both on the existing literature and our photocatalytic experimental results. More references could be provided, if required by the reviewer.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript reports on the preparation and characterization of Cu- and Ag-doped TiO2 powders, as well as degradation experiments of methylene blue solutions. The manuscript is well organized and logically structured into sections. There are points that require further clarification based on relevant literature. Moreover, a consistent presentation of the experimental results in plots is required as indicated in the attached pdf file, contrary to various presentation of figures presented in the manuscript.

Comments for author File: Comments.pdf

Author Response

Reviewer #3

The manuscript reports on the preparation and characterization of Cu- and Ag-doped TiO2 powders, as well as degradation experiments of methylene blue solutions. The manuscript is well organized and logically structured into sections. There are points that require further clarification based on relevant literature. Moreover, a consistent presentation of the experimental results in plots is required as indicated in the attached pdf file, contrary to various presentation of figures presented in the manuscript.

1. Line 68-75: Indeed, addition of metals may increase the bandgap of TiO2, however the purity and nanostructure of the films is detrimental for efficient photocatalysis to suspend non radiative recombination and has been widely reported in several reports, e.g.

https://doi.org/10.1021/acsami.6b05135

https://doi.org/10.1039/C7CP06742A

https://doi.org/10.1504/IJNT.2014.064297

A clear introduction of this point can educate the reader on the challenges of making an efficient photocatalyst.

Moreover, the last sentence appears incomplete. Please add and clarify to address these concerns.

Reply: We would like to thank the reviewer for the suggested articles that improved the literature review of the manuscript. The suggested literature was added to the text for clarification (Lines 77-80 in the revised manuscript, references: 19-21). The sentence was also completed (Line 82 in the revised manuscript).

“On the other hand, the purity and nanostructure of the films are detrimental to efficient photocatalysis to suspend non-radiative recombination according to numerous literature reports [19-21].”

2. Line 68 misspelling

Reply: We would like to thank the reviewer for this comment that permits us to improve the presentation of the manuscript. The mistaken word was corrected (Line 71 in the revised manuscript).

3. Lines 85-92: The optimum concentration of Cu may depend on the composition of the matrix itself as controlled by the preparation/deposition method. Sol-gel methods reported different optima compared to sputtering methods in https://doi.org/10.1021/acs.jpcc.6b03058

and

https://doi.org/10.1016/j.apcatb.2014.12.053

Therefore, clarification is required to avoid any misconceptions to the reader.

Reply: We would like to thank the reviewer for the proposed articles that will prevent any misconceptions to the reader. The suggested literature was added to the text and discussed for clarification (Lines 100-105 in the revised manuscript, references: 28, 29).

“The optimum concentration of copper could possibly depend on the structure and the properties of the matrix material, while the preparation or deposition approach also affects and determines the properties of the composite materials.; thus, sol-gel methods report different optima, in comparison with the sputtering methods [28, 29].”

4. Lines 106-107: More details on the mechanism that increased the surface area would be helpful to the reader of the manuscript. Moreover, there should be a relation between a broader absorption of the codoped catalyst compared to a singly doped one and are expected to be introduced here.

Reply: We would like to thank the reviewer for this comment that improves the content of the manuscript’s literature review. The text was revised according to the reviewer’s suggestions (Lines 119-127 in the revised manuscript).

“The co-catalysts’ photocatalytic degradation of MB and salicylic acid under visible irradiation was approximately 2-4 times higher, compared to their monometallic counterparts as the co-catalysts exhibited better synergistic effect, increased lifetime of the generated charge carriers and active sites on the co-catalysts surface. The dual role of Ag and Cu metals resulted in enhanced photocatalytic performance in co-catalytic systems due to the combined functional behavior of deposited metals and improved interfacial charge transfer process.[33].”

5. Line 147: The figure was not mentioned in the main text of the mansucript.

Reply: We would like to thank the reviewer for spotting this omission. Figure 1 is now mentioned in the text (Line 176 in the revised manuscript).

6. Line 159-160: What is the specrum of the LED? Adding this specification can provide a better undestanding of the effective range of the catalyst.

Reply: We would like to thank the reviewer for the comment. The light spectrum of the lamp was added in the text (Line 194 in the revised manuscript).

“a Vis LED lamp (30 W E27, 230 V, wide beam 200°, cold white light 6200k, light intensity 3500lux Ra>80, Pf>0.5, Energy class A+) with wavelength range 400-700nm”

7. Line 193: The peaks cannot be clearly seen. Please provide a figure of higher resolution. In the interest of space the two XRD patterns can be plot in the same graph and the Y axis can also be added. A reference with reported XRD data is also required to confirm the phases TiO2. The XRD of the Cu- and Ag-doped TiO2 and mixed phases would be more useful.

Reply: We would like to thank the reviewer for this comment that improves the appearance of the manuscript. The figures were replaced (Lines 229-230 in the revised manuscript) and an explanatory sentence regarding the XRD analysis of the photocatalysts was added to the text (Lines 234-237 in the revised manuscript). A reference regarding the XRD spectrum peaks of the titania phases was added in the article (Reference 38 in the revised manuscript).

“The metal-doped catalysts were also analyzed by XRD but provided spectra identical to their corresponding substrate. This is might be attributed to the low metal concentration of the examined photocatalysts which is below the detection limit of the instrument (~3%).”

8. Lines 199-201: A rationale for the selected metal loading and rutile/anatase substrates was not provided. Please explain.

Reply: We would like to thank the reviewer for this comment that permits us to enhance the reader’s understanding. Low metal loadings on titania result in more catalytically effective photocatalysts. According to our experiments, the 1% metal loading was not photocatalytically beneficial for either Ag or Cu prepared photocatalysts, and their optima ranged between 0.1-0.5%. That is why we tested this range. An explanatory sentence was added to the text (Lines 239-241 in the revised manuscript).

“The metal concentration on the titania substrate that was investigated was 0.1-1% while commercially available titania rutile and anatase were employed in the photocatalysts preparation. Low metal loadings ranging between 0.1-1% were  investigated.”

9. Lines 208-209: Please define the acronyms BET and BJH on their first instance.

Reply: We would like to thank the reviewer for this comment. The acronyms explanation was added in the text (Lines 254-256 in the revised manuscript)

10. Line 220: Please consider plotting the classified isotherms as a reference along with the measured to confirm the pore morphology.

Zooming to 0.8-1.0 range can further indicate the minimal hysteresis of the sample.

An explanation for the presenting only this sample is also required.

Reply: We would like to thank the reviewer for this comment. Type II isotherm curve was added in the text (Line 267 in the revised manuscript). Regarding the explanation, it is mentioned in Lines 257-258 (revised manuscript): “All the obtained isotherms were similar to the one that is described in Figure 5b”.

11. Line 234: E-171 was not defined.

Reply: We would like to thank the reviewer for spotting this mistake and permitting us to improve our manuscript. The mistaken sentence in the text was erased (Lines 285-286 in the revised manuscript).

12. Lines 244-251: the irradiation was defined as visible in the title and methods, but now UV is mentioned. Please see also comment in methods on the spectrum of the source.

Please also ensure bold is required otherwise revise.

Reply: We would like to thank the reviewer for this comment. The mistake in the text was corrected (Lines 299-300 in the revised manuscript), the bold is revised and the spectrum of the source was added to the text (Line 194 in the revised manuscript).

13. Lines 275-277: Did the authors consider why rutile is more efficient than anatase? A discussion based on the vast existing literature on TiO2 is missing at this point.

Reply: We would like to thank the reviewer for this comment that permits us to further support our results with relevant literature. A possible explanation was added to this point according to the reviewer’s comment (Lines 332-335 in the revised manuscript).

“Titania anatase has a 3.2eV energy band gap that is higher that this of the Rutile TiO2 (3.0eV). [44] Since the metal addition further decreases the photocatalysts’ energy band gap, it could possibly justify its enhanced efficiency towards the MB degradation under visible light.”

14. Lines 285-286 Figure 8: It is not clear why a degradation plot is presented instead of MB concentration as previously presented. A consistent presentation is required to enable meaningful comparisons between Cu- and Ag-doped samples.

Reply: We would like to thank the reviewer for the comment. Figure 8 was revised according to the reviewer’s comment (Lines 345-346 in the revised manuscript).

15. Line 292: Paragraph number is missing.

Reply: We would like to thank the reviewer for the comment. The number of the paragraph was added (Lines 355 in the revised manuscript).

16. Lines 297-298 Figure 9: The presentation can be confusing to the reader.

A consistent figure style is suggested.

Please see also previous comment on figure 8.

Reply: We would like to thank the reviewer for this comment that led to the better presentation of the experimental results. Figure 9 was revised according to the reviewer’s suggestion (Line 361-362 in the revised manuscript).

16. Line 301: Misspelling

Reply: We would like to thank the reviewer for the comment. The misspelling was corrected (Line 366 in the revised manuscript).

17. Line 302: Wrong word

Reply: We would like to thank the reviewer for the comment. The phrase was revised (Line 367 in the revised manuscript).

18. Lines 313-314, Figure 10: What is the content of anatase in the samples? A secondary X axis at the top of the graph for the anatase content, can address this concern.

Reply: We would like to thank the reviewer for the comment that permits to improve the reader’s understanding. A sentence that clarifies the percentage of rutile and anatase is added in the text (Lines 372-374 in the revised manuscript).

“In each experiment, the 100% substrate was completed by adding the rutile and ana-tase percentage, e.g. in the experiment with 70% rutile content, the other 30% is titania anatase.”

19. Lines 316-319: The 4h to 1.45h reduction is not shown in the figure. Do the authors refer to another figure or to results not shown in the manuscript? A clarification is required.

Reply: We would like to thank the reviewer for the comment. The sentence was revised, and the compared figures were added in the sentence for better understanding (Lines 386-387 in the revised manuscript).

“More specifically, in the case of Cu-based photocatalyst, the catalyst prepared with 5% anatase and 95% rutile achieved a major decrease in the irradiation time required to achieve 90% MB degradation (from 2.5h to 1h), comparing the results of Figure 7 and Figure 10.”

20. Lines 324-329: Were the same methods used for the preparation of the samples in other studies? A discussion on the preparation methods for photocatalysts is missing here and can suggest means for further research.

Reply: We would like to thank the reviewer for the comment. The text was revised according to the comment and the preparation methods are mentioned in the text (Lines 395-397 and 402-404 in the revised manuscript).

“In this study, the preferential Ag deposition on the anatase/rutile interface was held responsible for the superior photocatalytic activity in the case of the anatase and rutile mix substrate. [46].”

“In their approach though the Sm-doped titania photocatalysts were prepared via a sol-gel approach using precursor compounds for the metal and the titania synthesis [47].”

21. Lines 344-346: Can further studies elucidate this hypothesis? Specific suggestions can be added for the benefit of the reader.

Reply: We would like to thank the reviewer for the comment. Suggestions for further studies were added to the text to confirm the hypothesis that the metal copper particles act as sites for recombination of the generated electron holes and, hence, decrease the photocatalytic activity (Lines 423-426 in the revised manuscript).

“Further studies on this matter will be beneficial to confirm this hypothesis. Photoluminescence studies on the produced photocatalysts, as well as measurements UV–Vis diffuse reflectance spectra of the photocatalysts for the determination of their energy band gap will provide more data to further elaborate this finding.”

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Accept in present form

Author Response

The authors wish to thank the reviewer for this kind revision and accepting the manuscript in the present form.

Reviewer 2 Report

I don't think the authors answered my questions very well and provide enough evidence to support the conculsion. 

Author Response

Dear Reviewer, 

The authors would like to thank you for reconsidering the manuscript for publication, although according to your revision comments still needs to be revised. 
According to the authors, the main target of the this manuscript was to introduce a new method of catalyst developement, rather than describing and validating basic principles concerning photocatalytic reaction and performance . 
We would kindly aks you to reconsider your opinion, about defecient explanation and justification of the results . 

We are looking forward to your  opinion again 

Reviewer 3 Report

The manuscript has been thoroughly revised. The points of concern have been fully addressed and the results are now consistently and more clearly presented. The manuscript can be published.

Author Response

The authors wish to thank the reviewer for this kind revision and accepting the manuscript in the present form.