Interstitial N-Doped TiO₂ for Photocatalytic Methylene Blue Degradation under Visible Light Irradiation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study is devoted to the study of the properties of a catalyst modified with nitrogen atoms of interstinal origin. Undoubtedly, the use of photocatalysts as methods for cleaning organic substances is a very promising direction. The use of titanium oxide has a number of limitations, in particular, activation only in the ultraviolet range. Undoubtedly, chemical modifications of this catalyst, improving its activity in a wider range of conditions is a promising direction, which is the subject of this work.

The work describes well the obtaining a modification of the catalyst and applies adequate methods for testing the properties of the new catalyst.

As a wish to the authors - a presentation in the form of a diagram of the activation mechanism of the newly synthesized catalyst and its advantages over the original analogue.

As a discussion, I would like to clarify with the author. According to the literature, there are conflicting data on the toxicity of titanium oxide. Is it planned to conduct similar studies with a modified photocatalyst in order to assess the biosafety of modified titanium oxide in relation to living organisms.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Authors report an interstitial nitrogen-doped TiO2 (5%NT/TiO2) used thiourea as the N sourceisfabricated for the visible light derived MBdegradation. 5%NT/TiO2exhibitsan extended visible light absorption range. Besides, photoelectrochemical measurement shows an improvement in photocurrent response and charge transfer behavior on N/TiO2. Thus, 5%NT/TiO2hasenhanced photocatalytic activity compared to pristine TiO2 and substitutive N-doped TiO2(5%NAB/TiO2).The accelerated photocatalytic MB degradation process on N/TiO2 should be mainly attributed to the interstitial N-doping, which causes the appearance of new energy states and extended optical properties.

The following issues need to be addressed before publication

 1. Elemental mapping is missing. Authors should provide the elemental mapping of the TiO2 (5%NT/TiO2) sample.

2.  Why did the authors used TiO2?. TiO2 has been used for a long time and many reports are available with better performance.  Authors need to state the novelty of this work clearly.

3. More structure analysis is needed, therefore, I recommend authors to provide RAM analysis.

4. The authors need to state the advantage of using thiourea as a nitrogen source.

5. The Mott-Schottky diagram of of 5%NT/TiO2and 5%NAB/TiO2 is missing.

6. Authors need to check grammatical and typing errors

 

7. Authors need to compare their work with the recently reported catalysts. 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Authors need to check grammatical and typing errors

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript deals with the preparation of nitrogen-doped TiO₂ photocatalysts using two different N sources (thiourea and ammonium bicarbonate). The optimized material (5%N_T/TiO₂) showed the highest methylene blue (MB) degradation rate. However, there are many disadvantages that prevent me from considering this work for publication in Catalysts:

1. There are many works devoted to the investigation of nitrogen-doped TiO₂ photocatalysts. Therefore, there is a lack of novelty. In addition, there is no comparison with previous works on this topic.

2. The results obtained even for MB degradation are unconvincing. After 150 min, only 55% of MB was removed (with adsorption of about 35%). The proposed catalyst cannot be considered as highly efficient.

3. There are no studies on the stability of the catalyst as well as the reaction mechanism.

4. Activity against multiple pollutants (not just MB) needs to be investigated.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

In this study, the authors synthesized N-doped TiO₂ and demonstrated photocatalytic degradation of methylene blue (MB) under visible light. However, N-doped TiO₂ is now well known visible-light sensitive photocatalyst. Therefore, I cannot find the novelty in this study. 

 

In addition, although results obtained from a series of experiments are correct, the discussion based on the results is primitive. 

 

Therefore, I think this paper is not acceptable at present. 

 

Here, I list my comments.

 

1. In this study, the authors mentioned that N element was doped in the form of interstitial N and oxygen substitution when thiourea and ammonium bicarbonate was used, respectively. However, the reason why such difference was observed was not explained. The authors should explain the detailed (or assumed) mechanism to cause such difference because it is scientifically interesting and significant. 

 

2. Even though the calcination condition was same, crystal phase of undoped TiO₂, 5%, 5%N_T/TiO₂, and 5%N_AB/TiO₂ were all different.  Similar to N-doping mechanism, the authors should explain why such difference was occurred. 

 

3. From N₂ adsorption-desorption isotherms, the authors concluded that sensitized samples had mesopores. However, I cannot understand how such mesopores were formed even though a template (such as surfactant) was not introduced in the precursor solution. The authors should explain the mechanism to introduce mesopores. 

 

4. In line 143-144, even though the authors wrote “The spectra of 5%N_T/TiO₂ and 5%N_AB/TiO₂ exhibit additional peaks at 1427-1430 cm^-1 and 1280-1295 cm^-1.”, I cannot find any peaks in the spectra of 5%N_T/TiO₂ and 5%N_AB/TiO₂.

 

5. Depending on the position of introduced N elements, the position of N-derived intermediate level between the band-gap and its DOS (density of states) should be changed and this change explains the difference of optical properties between 5%N_T/TiO₂ and 5%N_AB/TiO₂. Therefore, the authors should include the discussion based on the first-principles calculation. 

 

6. Although the authors concluded that interstitial doping of N is more effective than substitutive doping, the reason why interstitial doping brings superior photocatalytic activity is not explained. 

 

7. In the caption of Figure 3, alphabets (e.g. (a), (b), …) used to distinguish figures should be placed before the explanation such as “(a, d) TiO₂, (b, e) 5%N_T/TiO₂, and (c, f) 5%N_AB/TiO₂”.  

 

8. The color used to distinguish TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ should be consistent among all figures. In the submitted manuscript, although the data of 5%N_AB/TiO₂ is shown in Red in Figure 1, Green and Blue was used in Figure 4 (a) and (b). This is confusing. 

 

9. In Line 202, I think “40 min of” is correctly “40 mL of”. Similar mistakes are frequently shown in “3.1.1 5%N_T/TiO₂ preparation” section.

 

10. In “3.3 Catalyst evaluation” section, the authors should add the spec of Xenon lamp used in this study. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The manuscript is well written and offers some novelty in the already exensively researched field of photocatalysis. The citations are up-to-date, as well as the used methods. All in all, I believe this manuscript is ready for publishing.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have answered all my concerns correctly, and the paper can be accepted in its current form. 

Comments for author File: Comments.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Because the authors well revised based on my comments, I think the revised manuscript is now acceptable.