Facile Synthesis of Anatase–Rutile Diphase N-doped TiO₂ Nanoparticles with Excellent Visible Light Photocatalytic Activity

Round 1

Reviewer 1 Report

The authors presented a study on nitrogen doped TiO2 catalyst for organic photodegradation, looking at details such as annealing temperature and photocatalytic active species via introduction of scavengers. The manuscript can be published after minor revisions:

1. There are already many reports of nitrogen-doped TiO2, so the novelty factor is unclear and needs to be elaborated. For instance, is this the first time that sintering with TiN is reported? What is the significance of this method?
2. What is the reason that sintering at 600 deg C led to poor light absorption and hence poor photocatalytic activity? This needs to be explained.

Author Response

The authors presented a study on nitrogen doped TiO₂ catalyst for organic photodegradation, looking at details such as annealing temperature and photocatalytic active species via introduction of scavengers. The manuscript can be published after minor revisions:

1. There are already many reports of nitrogen-doped TiO₂, so the novelty factor is unclear and needs to be elaborated. For instance, is this the first time that sintering with TiN is reported? What is the significance of this method?

Thank you for your good suggestion. The novelty of this paper has been indicated in the revised abstract and introduction part. This is not the first report on the preparation of N-doped TiO₂ by sintering TiN powders. In the introduction part, we have mentioned that Sun et al. have fabricated the N-doped TiO₂ by calcining TiN under carbon monoxide atmosphere in a range of 400-600 ^oC [23]. Different from reference [23], we have obtained the N-doped TiO₂ by directly sintering TiN under the ambient atmosphere.

 

2. What is the reason that sintering at 600 deg C led to poor light absorption and hence poor photocatalytic activity? This needs to be explained.

Thank you for your good suggestion. The higher calcination temperature (~600 ^oC) will lead to a decline in the light absorption capacity of N-doped TiO₂ sample, which may be that the higher temperatures result in the complete oxidation of TiN and the oxygen atoms have totally replaced nitrogen atoms in the crystalline lattice. Comparing with other samples (N-TiO₂-400 and N-TiO₂-500), a poor photocatalytic activity of N-TiO₂-600 sample under visible light is due to its large band gap (3.0 eV), which can’t be excited by visible light.

Reviewer 2 Report

Article could be published after improving.

I have some suggestions for the authors

Current literature is noteworthy, as it shows that the subject matter is interesting for many research centers.
In the introduction, the authors cite the literature in a collective manner, eg: "To realize this goal, doping TiO2 with non-metal elements or constructing composites by coupling TiO2 with a narrow bandgap semiconductor is considered as the promising approaches [11-22]". Describe the referenced literature in more detail.
On line 41 we have [23-29]
On line 56 - [30-32]
On line 63 - [33-35]
please add more information what could be found in the mentioned references

in the Introduction, there is no information where the results of the research can be used.

There should be more concrete research results in the abstract

Author Response

In the introduction, the authors cite the literature in a collective manner, eg: "To realize this goal, doping TiO₂ with non-metal elements or constructing composites by coupling TiO₂ with a narrow bandgap semiconductor is considered as the promising approaches [11-22]". Describe the referenced literature in more detail.

On line 41 we have [23-29]

On line 56 - [30-32]

On line 63 - [33-35]

please add more information what could be found in the mentioned references in the Introduction, there is no information where the results of the research can be used. There should be more concrete research results in the abstract.

Thank you for your good suggestion. The details on the relevant references have been described in the introduction part. For example, Sun et al. have fabricated the nitrogen doped TiO₂ by calcining TiN under carbon monoxide atmosphere in a range of 400-600 ^oC. The enhanced photocatalytic activity toward the degradation of methylene blue over the obtained samples are achieved [26]. Wang et al. have prepared the nitrogen doped TiO₂ nanosheets by annealing TiO₂ nanosheets in NH₃ atmosphere with different NH₃ flow rates at 400 ^oC. The as-prepared N-doped TiO₂ photocatalysts show the remarkably boosted activity for degrading rhodamine B [27]. Yu’s group have synthesized nitrogen and sulfur co-doped TiO₂ nanosheets using thiourea as a dopant. The degradation performance of 4-CP aqueous solution under visible light is significantly improved [28].

Recently, some research groups have constructed the anatase-rutile phase junction photocatalysts by different synthetic approach, and they all present the enhanced photocatalytic performance in contrast with the single phase TiO₂ [33-38]. For instance, Yu’s group have designed and prepared the anatase-rutile TiO₂ nanofiber photocatalysts by electrospinning process, which are endowed the excellent photocatalytic H₂ evolution activities [33]. Qiu et al. have constructed the anatase-rutile heterojunction composites by a multiple synthetic route. The resultant photocatalysts present the enhanced photocatalytic H₂ evolution performance and pollutant degradation [34]. The superfine crystallinity TiO₂ particles with tunable anatase-rutile ratios have been synthesized via a solvothermal treatment method by Gao et al, which exhibits the remarkably improved photocatalytic activities toward organic pollutant degradation [35].  

Reviewer 3 Report

The work here reported is a nice study on the synthesis of an original TiO₂-based photocatalyst with good efficiency under visible light. The topic is suitable for Catalysts, and in general the manuscript is well structured and clearly written. It also handles a significant number of experiments. However, before it can be accepted for publication, the authors must implement several improvements. The two main ones are the following:

1) In the introduction, the authors present doping TiO₂ with non-metal elements (for example N) or constructing composites by coupling TiO₂ with a narrow band gap semiconductor as the promising (it sounds as only) approaches to enhance photoactivity of TiO₂ under visible lignt. What about doping with gold? The extensive literature related to gold-doped TiO₂ photocatalysts (especially in the last few years) does not deserve to be ignored in this way. Some mention in this section to this another family of photocatalysts and therefore the inclusion of some references dealing with Au-doped TiO₂ photocatalysts is mandatory. Moreover, a brief description of the pros and cons of each type should be also added to complete the state-of-art description.

2) In relation to the above, no comparison is made at all between the results obtained in the photodegradation of 4-CP and MB experiments in this work and the results obtained by other authors in the same reactions but employing different photocatalysts. This is necessary to provide the reader an idea about the real goodness and competitive character of the photocatalysts here prepared. In fact, all the references are just used for the introduction! This lack of bibliographic references is also evident in other parts of the text, for example in lines 121 (to illustrate the PL method), 130 (previous report on XRD study of P25), 132 (for equation 1), 167 (previous literature on XPS analysis) and 183 (previous reports on UV-vis reflectance spectra).

Additionally, there are many other corrections to be made:

a) English should be revised throughout the whole text. Some examples of typos and other related deficiencies are:

• line 32: efficiency
• line 33: carriers
• line 37: "the" must be deleted 
• line 51 "possesses higher"...
• line 55: "lest" should be "less"
• line 56: attention (no plural)
• line 64: "incorporing notrogen doping to the formation..."
• line 69: "stability of the photocatalyst"
• line 79: "temperature"
• line 81: "As reference" instead of "As control"
• line 136: "also increase" better than "improve" (referring to contents)
• line 152: "consistent with" instead of "in consistent with"
• line 188: "shown in Figure 4b inset"
• lines 194-195: "narrowest", "best" and "highest"
• lines 216 and 261: "follows the order" instead of "is according to the following order"
• Use of "respectively": delete it in line 212, insert it in line 238.
• line 225: similar-identical
• line 227: were (no was)
• line 248: "a comparative" (no "the comparative")
• line 250: "Considering that" (no "According to")
• use of "that": insert it in line 252 (after "suggesting"), delete it in line 258 (after "implying")
• etc, etc, etc.

b) Attention with subscripts and super indexes! (lines 20, 46, 78, 81, etc). Also letter size in line 165.

c) Line 42: Cui et al. should be Sun et al.

d) Some important experimental details are missing: samples pre-treatment before N₂ physisorption, and before XPS analysis to avoid charge effects, and step size and counting time in the XRD study.

e) Scherrer equation is so well-known (not only by catalytic community) that its inclusion is not needed.

f) Table 1: position of (110) is wrong, and two decimals for specific surface area values is excessive (also in lines 203-204)

g) Line 153: the authors should make explicit why they only show HRETM and XPS data of N-TiO₂-500. 

h) Line 162: the authors should explain where the C comes from.

i) Lines 177-178: the authors justify the absence of the two peaks at 446 and 612 cm^-1 in the N-TiO₂-200 sample spectra to its little amount of rutile phase, but the truth is that, according to Table 1, this content (18.3%) is very similar to that of N-TiO₂-500 (20.2). Any comment?

j) Line 198: in Figure 4d and Table 1.

k) Line 240: the correct redaction would be "are the most significant inhibitors of the photodegradation efficiency"

l) Figure 5.b caption: specify the sample to which the spectra correspond.

Author Response

The work here reported is a nice study on the synthesis of an original TiO₂-based photocatalyst with good efficiency under visible light. The topic is suitable for Catalysts, and in general the manuscript is well structured and clearly written. It also handles a significant number of experiments. However, before it can be accepted for publication, the authors must implement several improvements. The two main ones are the following:

1. In the introduction, the authors present doping TiO₂with non-metal elements (for example N) or constructing composites by coupling TiO₂with a narrow band gap semiconductor as the promising (it sounds as only) approaches to enhance photoactivity of TiO₂ under visible light. What about doping with gold? The extensive literature related to gold-doped TiO₂ photocatalysts (especially in the last few years) does not deserve to be ignored in this way. Some mention in this section to this another family of photocatalysts and therefore the inclusion of some references dealing with Au-doped TiO₂ photocatalysts is mandatory. Moreover, a brief description of the pros and cons of each type should be also added to complete the state-of-art description.

Thank you for your good suggestion. The relevant description has been provided in the introduction part. To realize this goal, surface modification by plasmonic metal nanoparticles (such as Au), constructing composites by coupling TiO₂ with a narrow bandgap semiconductor, and doping TiO₂ with non-metal elements are all considered as the promising approaches. Among them, loading plasmonic Au nanoparticles on the surface of TiO₂ can improve the photocatalytic activity under visible light due to its exceptional localized surface plasmon resonance (LSPR) effect. In addition, Au nanoparticles can act as electrons sink and active sites for photocatalytic reactions [11-13]. Formation of TiO₂-based composites by combining a narrow bandgap semiconductor can boost the photogenerated charge separation efficiency, and then increase the photocatalytic activity [14-22]. Nonmetal doping TiO₂ with C, N, S or B element is a feasible and convenient method, which has been widely explored in the past few years [23-25].

 

2. In relation to the above, no comparison is made at all between the results obtained in the photodegradation of 4-CP and MB experiments in this work and the results obtained by other authors in the same reactions but employing different photocatalysts. This is necessary to provide the reader an idea about the real goodness and competitive character of the photocatalysts here prepared. In fact, all the references are just used for the introduction! This lack of bibliographic references is also evident in other parts of the text, for example in lines 121 (to illustrate the PL method), 130 (previous report on XRD study of P25), 132 (for equation 1), 167 (previous literature on XPS analysis) and 183 (previous reports on UV-vis reflectance spectra).

Thank you for your good suggestion. Under the current retrieval conditions, we have found the limited articles on N-doped TiO₂ for the degradation of 4-CP solution. Therefore, we could not provide a comparison between our results and previous works. Some references have been cited at the appropriate positions in the manuscript.  

 

Additionally, there are many other corrections to be made:

1. a) English should be revised throughout the whole text. Some examples of typos and other related deficiencies are:

• line 32: efficiency
• line 33: carriers
• line 37: "the" must be deleted
• line 51 "possesses higher"...
• line 55: "lest" should be "less"
• line 56: attention (no plural)
• line 64: "incorporing nitrogen doping to the formation..."
• line 69: "stability of the photocatalyst"
• line 79: "temperature"
• line 81: "As reference" instead of "As control"
• line 136: "also increase" better than "improve" (referring to contents)
• line 152: "consistent with" instead of "in consistent with"
• line 188: "shown in Figure 4b inset"
• lines 194-195: "narrowest", "best" and "highest"
• lines 216 and 261: "follows the order" instead of "is according to the following order"
• Use of "respectively": delete it in line 212, insert it in line 238.
• line 225: similar-identical
• line 227: were (no was)
• line 248: "a comparative" (no "the comparative")
• line 250: "Considering that" (no "According to")
• use of "that": insert it in line 252 (after "suggesting"), delete it in line 258 (after "implying")
• etc, etc, etc.

Thank you for your suggestion. The above-mentioned points in the manuscript have been revised.

 

165. b) Attention with subscripts and super indexes! (lines 20, 46, 78, 81, etc). Also letter size in line 165.

Thank you for your suggestion. The above-mentioned points in the manuscript have been amended.

 

1. c) Line 42: Cui et al. should be Sun et al.

Thank you for your suggestion. The relevant expression has been revised.

 

1. d) Some important experimental details are missing: samples pre-treatment before N2 physisorption, and before XPS analysis to avoid charge effects, and step size and counting time in the XRD study.

Thank you for your suggestion. The relevant experimental details have been supplemented in the revised manuscript.

 

1. e) Scherrer equation is so well-known (not only by catalytic community) that its inclusion is not needed.

Thank you for your comments. To address the widest possible readers, providing the Scherrer equation is helpful to understand this paper.

 

1. f) Table 1: position of (110) is wrong, and two decimals for specific surface area values is excessive (also in lines 203-204)

Thank you for your suggestion. The position of (110) plane is aligned, and one decimal place for specific surface area values has been adopted.

 

500. g) Line 153: the authors should make explicit why they only show HRETM and XPS data of N-TiO₂-500.

Thank you for your comments. The N-TiO₂-500 sample has the optimal photocatalytic activity, moreover the microstructures and compositions of different samples have little differences. Consequently, we have only provided the HRTEM and XPS data of N-TiO₂-500 sample.

 

1. h) Line 162: the authors should explain where the C comes from.

Thank you for your comments. The C 1s peak at 284.8 eV is due to the adventitious hydrocarbon from the XPS instrument itself.

 

18. i) Lines 177-178: the authors justify the absence of the two peaks at 446 and 612 cm^-1in the N-TiO₂-400 sample spectra to its little amount of rutile phase, but the truth is that, according to Table 1, this content (18.3%) is very similar to that of N-TiO₂-500 (20.2). Any comment?

Thank you for your comments. The two peaks intensities of rutile phase in the P25, N-TiO₂-500 and N-TiO₂-600 samples are all low, and only weak signals are detected. When the contents of rutile phase in the N-TiO₂-400 further decrease, it is difficult to inspect the rutile phase.

 

1. j) Line 198: in Figure 4d and Table 1.

Thank you for your suggestion. The relevant expression has been revised.

 

1. k) Line 240: the correct redaction would be "are the most significant inhibitors of the photodegradation efficiency"

Thank you for your suggestion. The relevant expression has been revised.

 

5. l) Figure 5.b caption: specify the sample to which the spectra correspond.

Thank you for your suggestion. The relevant expression has been revised.

Round 2

Reviewer 2 Report

In my opinion, the paper is ready to be published

Reviewer 3 Report

The authors have taken into account most of my comments and requests. As a consequence, it has been improved in a significant way. In my opinion, it can be accepted now for publication.