Refractory Metal Oxide–Doped Titanate Nanotubes: Synthesis and Photocatalytic Activity under UV/Visible Light Range
Round 1
Reviewer 1 Report
In this manuscript, the author presented a metal doping method of titanate nanotube to increase the photocatalytic ability towards MB and RhB removal. Detailed characterization and tests are conducted and the result is promising. I would like to recommend this article to be published on Catalysts if my concerns are addressed.
- For figure 1, XRD of non-doped titanate nanotube should be included.
- What is the element distribution on the doped metal on the titanate nanotube? Maybe EDS experiment could be done to provide this information.
- What is the dark dot in Figure 3a? Are they metal or metal oxide nanoparticles?
- Pre- and post- chacterization on the catalysts should be done to test the stability of this photocatalyst.
Author Response
Point 1: For figure 1, XRD of non-doped titanate nanotube should be included.
Response 1: As the reviewer suggested, we have revised Figure 1 to include the XRD spectra of un-doped titanate nanotubes, please see the attachment.
Point 2: What is the element distribution on the doped metal on the titanate nanotube? Maybe EDS experiment could be done to provide this information.
Response 2: As the reviewer suggested, we conducted EDS analysis for W-doped TNTs, as shown below. Consequently, it was confirmed that W atoms were uniformly distributed over the entire area of the TNTs. Please see the attachment.
Point 3: What is the dark dot in Figure 3a? Are they metal or metal oxide nanoparticles?
Response 3: The dark dots in Figure 3a in the previous version of the manuscript denote the remaining TiO2 nanoparticles that did not sufficiently react with NaOH. We believe this may confuse the readers; therefore, we have included a more legible TEM image, please see the attachment.
Point 4: Pre- and post- chacterization on the catalysts should be done to test the stability of this photocatalyst.
Response 4: We appreciate the reviewer’s insightful comments. We examined the stability of the synthesised TNTs by thermal analysis, please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript entitled "Refractory Metal Oxide – Doped Titanate Nanotubes: Synthesis and Photocatalytic Activity under UV / Visible Light Range" is devoted to the doping of hydrogen titanate nanotubes with refractory metal ions. The topic is not new and has been studied for a long time. The article can be accepted into the journal, however, it could be improved, given the fact that one of the authors is a co-author of the pioneering work in the field of synthesis of hydrogen titanate nanotubes.
Here are some questions that I would like to receive answers to:
- The introduction should be extended by pointing out the works in which hydrogen titanates were synthesized by the hydrothermal method and, indicating the advantages of this method over others.
For example, the works https://pubs.acs.org/doi/10.1021/am2008568, https://doi.org/10.1016/j.jcis.2017.03.075 would be helpful.
It would also be useful to analyze the work on the doping of hydrothermally synthesized hydrogen titanate
See, for example, work https://doi.org/10.1039/D0NA00932F
- Paragraph 2.3 specifies the UV light source, but does not specify the visible light source. Please indicate the light source.
- It is necessary to provide XRD spectra of pure undoped hydrogen titanate. Please mark the peaks on the spectrum with Miller's indices. Also, for confirmation, it would be correct to submit the card number indicating the XRD base.
In this work, used the technique described by one of the co-authors in the work [https://doi.org/10.1021/la9713816]. However, both in this work and in the previous work, where the authors synthesized hydrogen titanate nanotubes doped with Nb [https://doi.org/10.1016/j.apsusc.2016.08.132], hydrothermal treatment led to the formation of the anatase phase. What is not observed in the presented manuscript. How can this be explained?
- I would also like to see images of pure undoped hydrogen titanate. This also applies to SEM and TEM.
- In lines 113-114, the authors write: "... the Mo-TNT sample had a microstructure in which the TiO2 nanoparticles were dispersed around the aggregated nanotubes ...". First, it is unclear on what basis the authors conclude that these are precisely TiO2 nanoparticles. Second, even if we assume that these are TiO2 nanoparticles, it is not clear why they are formed only in the Mo-TNT sample. Nothing is said about this. How can this be explained?
- The study lacks any evidence for the presence of Mo6 +, and W6 + ions in the crystal structure of hydrogen titanate. EDX can show the fundamental presence of elements, XPS will show exactly what state they are in, and Raman spectroscopy can help identify whether Ti3+ is substituted by Mo6+, and W6+, or intercalation occurs into the interlayer space through ion exchange with H+/Na+. I recommend that you provide evidence.
- How correct is it when comparing the band gap of hydrogen titanate to refer to works that indicate the band gap of TiO2 in the form of anatase? I suggest replacing references 20 and 21 with https://doi.org/10.1039/D0NA00932F
- Please explain the meaning of lines 138-139 "... The Mo-TNTs and W-TNTs exhibited adsorption-degradation transition at ~ 30 min both under UV and visible irradiation in the MB solution ...". How was the adsorption-degradation transition determined under conditions of UV and visible irradiation with light?
The work does not say anything about dark adsorption experiments. They would be helpful though.
- In addition, dyes are quite actively degraded by UV-visible light. I recommend including photolysis data in order to understand how strong the effect of photocatalysts on the degradation process is.
Comments for author File: Comments.pdf
Author Response
Point 1: The introduction should be extended by pointing out the works in which hydrogen titanates were synthesized by the hydrothermal method and indicating the advantages of this method over others.
Response 1: We appreciate the reviewer’s suggestion. To address the reviewer’s comment, we have modified the introduction to provide the information and emphasis the benefits of hydrothermal methods. Please see the attachment.
Point 2: Paragraph 2.3 specifies the UV light source, but does not specify the visible light source. Please indicate the light source.
Response 2: To address the reviewer’s comment, we have specified the visible light source to provide the following information:
(Experimental procedure, paragraph 2.3).
Visible light (solar simulator with a cutoff glass filter under 400 nm, HAL-C100, Asahi spectra, Japan)
Point 3: It is necessary to provide XRD spectra of pure undoped hydrogen titanate. Please mark the peaks on the spectrum with Miller's indices. Also, for confirmation, it would be correct to submit the card number indicating the XRD base.
Response 3: To address the reviewer’s comment, XRD data of non-doped titanate nanotubes and Miller’s index were added to Figure 1. In addition, the JCPDS card number has been added to the manuscript. Please see the attachment.
Point 4: I would also like to see images of pure undoped hydrogen titanate. This also applies to SEM and TEM
Response 4: To address the reviewer’s comment, SEM and TEM images of non-doped titanate nanotubes were added to Figures 3 and 4, respectively. Please see the attachment.
Point 5: In lines 113-114, the authors write: "... the Mo-TNT sample had a microstructure in which the TiO2 nanoparticles were dispersed around the aggregated nanotubes ...". First, it is unclear on what basis the authors conclude that these are precisely TiO2 nanoparticles. Second, even if we assume that these are TiO2 nanoparticles, it is not clear why they are formed only in the Mo-TNT sample. Nothing is said about this. How can this be explained?
Response 5: The dark dots in Figure 3a in the previous version of the manuscript denote the remaining TiO2 nanoparticles that did not sufficiently react with NaOH. We believe that this may confuse the readers; therefore, we have revised the manuscript to provide a clear TEM image, please see the attachment.
Point 6: The study lacks any evidence for the presence of Mo6 +, and W6 + ions in the crystal structure of hydrogen titanate. EDX can show the fundamental presence of elements, XPS will show exactly what state they are in, and Raman spectroscopy can help identify whether Ti3+ is substituted by Mo6+, and W6+, or intercalation occurs into the interlayer space through ion exchange with H+/Na+. I recommend that you provide evidence.
Response 6: To address the reviewer’s comment, high-resolution XPS spectra of pure TNT, Mo-TNT, and W-TNT, which confirm the presence of Ti 2p, O 1s, W 4f, and Mo 3d, are shown in Figure 2. Please see the attachment.
Point 7: How correct is it when comparing the band gap of hydrogen titanate to refer to works that indicate the band gap of TiO2 in the form of anatase? I suggest replacing references 20 and 21 with https://doi.org/10.1039/D0NA00932F
Response 7: To address the reviewer’s comment, References 20 and 21 were replaced (in line 151) with https://doi.org/10.1039/D0NA00932F
(Manuscript, in lines 155–157)
‘The band gap energy of the pure TNTs (3.34 eV) analysed was consistent with the results of other research studies [35], and the Mo- and W-TNTs were shown to have a band gap energy similar to that of pure TNTs’.
* Reference 35 (https://doi.org/10.1039/D0NA00932F)
Mendez-Galvan M., Celaya C. A., Jaramillo-Quintero O., Muniz J., Diaz G., Lara-Garcia H. A., Tuning the band gap of M-doped titanate nanotubes (M=Fe, Co, Ni, and Cu):an experimental and theoretical study, Nanoscale Adv. 2021, 3, 1382-1391.
Point 8: Please explain the meaning of lines 138-139 "... The Mo-TNTs and W-TNTs exhibited adsorption-degradation transition at ~ 30 min both under UV and visible irradiation in the MB solution ...". How was the adsorption-degradation transition determined under conditions of UV and visible irradiation with light?
Response 8: To address the reviewers comment, we have revised manuscript in lines 178-182.
(Manuscript, in lines 178-182)
“The Mo-TNTs and W-TNTs exhibited faster degradation at ~30 min both under UV and visible irradiation than pure TNT and P25 in the MB solution. In particular, that degradation by refractory metal-doped TNTs were increased significantly under UV and visible light irradiation compared with that of the pure TNTs.”
Point 9: In addition, dyes are quite actively degraded by UV-visible light. I recommend including photolysis data in order to understand how strong the effect of photocatalysts on the degradation process is.
Response 9: We consider that the degradation of cationic dyes (MB and RhB solutions) might not be significant under UV-visible light in the present study. As the reviewer stated, the degradation of cationic dyes has sometimes been reported with copper-based photocatalysts, mainly originating from the reaction between dyes and the copper-based photocatalyst under UV-visible light [Ⅰ]. By contrast, the direct photolysis in the absence of photocatalysts showed insignificant conversion of the cationic dyes under UV-visible light [Ⅱ].
Ref. Ⅰ. RSC Adv., 2020, 10, 37028-37034 [https://doi.org/10.1039/D0RA05275B]
Ref. Ⅱ. Journal of Colloid and Interface Science 356 (2011) 211–216 [https://doi.org/10.1016/j.jcis.2010.12.059]
Author Response File: Author Response.pdf
Reviewer 3 Report
The article describes the synthesis of TNT nanotubes. The work is quite interesting, but it is written somewhat carelessly, a number of comments arise.
- The introduction should be substantially extended, and references should be made to recent work on titanium dioxide nanotubes [DOI: 10.1016/j.enmm.2021.100537; DOI: 10.1016/j.jallcom.2019.04.342; DOI: 10.1016/j.jpowsour.2021.230274].
- The given characterization does not provide any data on the state of molybdenum and tungsten. Investigations by the XPS method, as well as high-resolution TEM, are required to determine the oxiadation state and morphology of the particles of these metals.
- It is necessary to describe in detail the properties of the light sources.
- The mechanism of the activity increase should be provided in some schematic form.
- Stability experiments should be carried out.
Author Response
Point 1: The introduction should be substantially extended, and references should be made to recent work on titanium dioxide nanotubes [DOI: 10.1016/j.enmm.2021.100537; DOI: 10.1016/j.jallcom.2019.04.342; DOI: 10.1016/j.jpowsour.2021.230274].
Response 1: As the reviewer suggested, we have revised the introduction to provide a detailed review of recent work on titanium dioxide nanotubes, including the suggested references, please see the attachment.
Point 2: The given characterization does not provide any data on the state of molybdenum and tungsten. Investigations by the XPS method, as well as high-resolution TEM, are required to determine the oxidation state and morphology of the particles of these metals.
Response 2: To address the reviewer’s comment, high-resolution XPS spectra of pure TNT, Mo-TNT, and W-TNT, which confirm the presence of Ti 2p, O 1s, W 4f, and Mo 3d, are shown in Figure 2. Please see the attachment.
Point 3: It is necessary to describe in detail the properties of the light sources.
Response 3: To address the reviewer’s comment, we have specified the visible light source to provide the following information:
(Experimental procedure, paragraph 2.3).
Visible light (solar simulator with a cutoff glass filter under 400 nm, HAL-C100, Asahi spectra, Japan)
Point 4: The mechanism of the activity increase should be provided in some schematic form.
Response 4: As the reviewer suggested, we have added a schematic depiction to the manuscript to describe the mechanism of the activity increase, please see the attachment.
Point 5: Stability experiments should be carried out.
Response 5: We appreciate the reviewer’s insightful comments. We examined the stability of the synthesised TNTs by thermal analysis, please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I would like to thank the author's effort on addressing my concerns. I just have two more questions about the author's reply.
1) For Figure 1, why the peak at ~10 degree for TNT did not show in the XRD patterns of Mo-TNT and W-TNT?
2) For my previous comments on pre- and post- characterization of the photocatalysts. I appreciated that the author provide solid data to show that the catalyst is thermally stable. However, I was actually asking about pre- and post- photocatalytic reaction chacterization on the catalysts. Is the photocatalyst stable after the photocatalytic reaction?
Author Response
Point 1:For Figure 1, why the peak at ~10 degree for TNT did not show in the XRD patterns of Mo-TNT and W-TNT?
Response 1: We appreciate the reviewer’s insightful comment. We consider two reasons that XRD peak at ~ 10 degree did now show in Mo-TNT and W-TNT.
- Mozia et al. reported that loading of Ag on the surface TNTs, fabricated by a hydrothermal method, may weaken the intensity of peak corresponding to the (200) plane at 2. The peak seemed to almost disappear as the content of Ag exceeded 2.5 mM. This was because during the synthesis of Ag/TNTs, Ag + ions first diffused to the TNTs’ surface and deposited as silver hydrate intermediate (Ag(OH)n (H2O)m, which upon dehydration with surface Ti–OH groups resulted in binding to the surface by sharing with surface oxygen atoms of the TiO6 octahedron layers in the (200) planes of the TNTs. As a result, the (200) planes of the TNTs underwent drastic deformation, and hence loss of its X-ray diffraction pattern with Ag loading. According to the these mechanism, we considered that the XRD peak at 2disappeared in Mo- and W-TNTs.
- We also consider that the peak for the (200) plane of TNTs might not be clearly detected when its intensity is small (for the case of Mo- or W- doped TNTs) because of the significant background noise (swelling of the spectra) typically presenting in the extremely low 2 theta ranges.
The manuscript has now been revised to provide this clarification as shown below:
(Manuscript, in lines 110-117)
In addition, the peak for the (200) plane at 2~10 was not present in the XRD patterns of Mo- and W-TNTs. Loading of Mo or W on the surface of TNTs may deform the crystalline structure of the surface layer, weakening the intensity of the peak for the (200) plane [32, 33]. Furthermore, the peak for the (200) plane of TNTs might not be clearly detected when its intensity is small (for the case of Mo- or W- doped TNTs) because of the significant background noise (swelling of the spectra) typically presenting in the extremely low 2 theta ranges.
Point 2: For my previous comments on pre- and post- characterization of the photocatalysts. I appreciated that the author provide solid data to show that the catalyst is thermally stable. However, I was actually asking about pre- and post- photocatalytic reaction chacterization on the catalysts. Is the photocatalyst stable after the photocatalytic reaction?
Response 2: We apologize for misunderstanding the reviewer’s comments. As a reviewer’s comment, it is necessary to evaluate the stability of the photocatalyst for recycling. However, in case of metal ion-doped TNTs, it has been reported that structural change is not significant in dyes such as methylene blue (MB) or rhodamine B (RhB) [Ⅰ, Ⅱ]. Please excuse again that it is difficult to characterise the TNTs after testing the photocatalytic activity in the current test system within the given deadline (a day for the second revision).
Ref. Ⅰ. H. Wang et al., Power Technology, 2018, 326, 272-280.
[https://doi.org/10.1016/j.powtec.2017.12.010]
Ref. Ⅱ. J. Wang et al., Nanomaterials, 2020, 10, 1345.
[http://dx.doi.org/10.3390/nano10071345]
Author Response File: 
Author Response.docx
Reviewer 2 Report
The authors have made changes in accordance with the comments. I believe that the manuscript can be published
Author Response
The authors have made changes in accordance with the comments. I believe that the manuscript can be published
Reviewer 3 Report
I'm satisfied with the review.
Author Response
I'm satisfied with the review.
