Hexavalent Chromium Removal via Photoreduction by Sunlight on Titanium–Dioxide Nanotubes Formed by Anodization with a Fluorinated Glycerol–Water Electrolyte

Round 1

Reviewer 1 Report

The objective of this research is clear and the results are novel and important. The manuscript is written in proper English. Despite that, the manuscript has some weak points and needs serious revision.

The experimental part should be put between the Introduction and the Results, and not placed after the Results and Discussion.

The results and discussion should be separated. Now it is difficult to understand the physical, chemical, and electrochemical mechanisms that originate in observed relationships between process parameters and characteristics of nanotubes and chromium removal efficiency. The results section should compose of subsections, successively, on the effect of NH4F content, potential, and process time on nanotubes characteristics, the effect of annealing environment on nanotubes, the effect of an environment on CR(VI) removal efficiency, the effect of EDTA and that of Cr initial concentration. The discussion can be separated into the processes determining the nanotubes' characteristics, and those determining the Cr removal efficiency. Nowi t is difficult to understand the concurrent processes and their importance. On the other hand, the discussion should be more precise and deepened.

Figure 3, line 155: what was the content of fluoride (no data in a caption); please explain why such amount was selected? I cannot find such information.

Figure 4, line 171: what was the content of fluoride and polarization time (no data in a caption).

Figure 5, line 196: what was the content of fluoride (no data in a caption).

Figure 6, line 207: what was the content of fluoride (no data in a caption).

Line 135: There is also a minimum voltage at which the TNT structure forms; please give a reference and explain why it is so.

In the experimental part, there are no characteristics of the used power supply. Besides, please give the full characteristics of all purchased chemicals, such as the chemical purity, delivering company, and country, and for the equipment not only the name. But also the company and country.

The conclusions are limited only to summarizing the results and not to their scientific explanation. They should be rearranged to show the relationships and justify them.

The change of Paste Tense to the Present Perfect is advised for sentences shown by color in lines 106-108, 110-112, 214-216, 228-229.

Some sentences or phrases are unclear or inappropriate:

Line 225: The average crystallite size is 26.29 and 36.79 nm; rather `crystallite sizes were …`

Line 64: fluoride ions have been identified as the most important parameter; rather `a presence of …`

Line 122: oxide follows a flow mechanism in pore formation; unclear.

Line 135: electric field dissolution; rather `electric-field enhanced …`

Line 164-166: at this voltage, the anodic oxide underwent severe polarization, which led to dissolution and the formation of an irregular porous structure.; anodic polarization, I presume? Dissolution of what? What does it mean, a flow mechanism in pore formation, flow of what? Be more precise.

Line 234: The…function multiplied by … using a corresponding coefficient (n); unclear.

Line 235: The n values of 0.5 and 2 used to estimate direct and indirect bandgap oxide; rather `…were used…`

Line 294-295: Reduction experiment (Figure 11) shows reduction was very fast; suggested `The experiment showed very fast reduction` (of what?)

Line 329: 111Table 2; delete 111.

Lines 332-333: adding a small amount of EDTA (1 mM) amounts under UV–Vis light irradiation, such as phenol (10 mM) and tartaric acid (6 mM) …; unclear.

Line 357: The anodized Ti foil was removed from the electrolyte and cleaned; how was it cleaned?

Line 383: proposed electrolyte is dependent on NH4F concentration, anodization duration, and applied voltage; both unclear and improper.

Line 144, 146: different loadings of NH4F; rather different contents or concentrations

Attention: all remarks are indicated in the revised manuscript by color.

Comments for author File: Comments.pdf

Author Response

Thank you very much for the effort and time in reviewing our work. The answers to the comments are as follows: 

Q1: The experimental part should be put between the Introduction and the Results, and not placed after the Results and Discussion.

Authors’ response: This manuscript was prepared according to the template from MDPI Catalysts whereby the experimental part is located after the Discussions and before the Conclusion.

Q2: The results and discussion should be separated. Now it is difficult to understand the physical, chemical, and electrochemical mechanisms that originate in observed relationships between process parameters and characteristics of nanotubes and chromium removal efficiency. The results section should compose of subsections, successively, on the effect of NH4F content, potential, and process time on nanotubes characteristics, the effect of annealing environment on nanotubes, the effect of an environment on CR(VI) removal efficiency, the effect of EDTA and that of Cr initial concentration. The discussion can be separated into the processes determining the nanotubes' characteristics, and those determining the Cr removal efficiency. Now it is difficult to understand the concurrent processes and their importance. On the other hand, the discussion should be more precise and deepened.

Authors’ response: Thank you for the suggestion; subsections a below, have been added. Elaboration and separated discussion involving the nanotubes’ characteristic and Cr removal efficiency are also given in the revised manuscript.  2.1.1. Effect of NH₄F concentration (Line 102)

2.1.2. Effect of anodization voltage (Line 136)

2.1.3. Effect of variation reaction time (Line 192)

2.1.4. Effect of annealing environment (Line 232)

2.2.1. Effect of EDTA as hole scavenger (Line 337)

Q3: Figure 3, line 155: what was the content of fluoride (no data in a caption); please explain why such amount was selected? I cannot find such information.

Authors’ response: The content of NH₄F used was 0.5 wt.% for the investigation of voltage effect. The explanation for the content of fluoride used is added and explained in subheading 2.1.1 Effect of NH₄F content (Page 3/20).

From here, 0.5 wt.% NH4F was chosen as the optimum amount of fluoride ion as it is adequate for surface etching, and the nanotubes formed are also with considerable length as shown in Figure 2. (Line 130-132)

The caption has been rewritten as follows:

Figure 3. FESEM images of anodized TiO₂ at different voltages: (a) 1 V; (b) 3 V; (c) 5 V; (d) 10 V; (e) 15 V; (f) 20 V; (g) 30 V; (h) 40 V; (i) 60 V and (j) 80 V. TEM images for the 20 V sample (k) bottom, and (l) to show serrated walls.  All anodization was done in glycerol + water + 0.5 wt% NH4F electrolyte for 90 min.  (Line 166-168).

Q4: Figure 4, line 171: what was the content of fluoride and polarization time (no data in a caption).

Authors’ response: The content of NH₄F used was 0.5 wt.% and anodization time was set at 90 min in this investigation. The information is added into the caption of revised manuscript at page 6/20 as follows:

Figure 4. Anodic film formation features (NH₄F = 0.5 wt.%, time = 90 min) at 1, 3, 5, 10, 15, 20, 30, 40, 60, and 80 V in a water–glycerol electrolyte. (Line 189-190).

Q5: Figure 5, line 196: what was the content of fluoride (no data in a caption).

Authors’ response: The content of NH₄F used was 0.5 wt.%. The information is added into the caption of Figure 5 in page 7/20 as follows:

Figure 5. FESEM images of the anodized TiO₂ at different anodization times for (a) 1 min; (b) 15 min; (c) 30 min; (d) 60 min; (e) 90 min; (f) 120 min; (g) 180 min; (h) 360 min and (i) 720 min.  All anodization was done at 20 V in glycerol + water + 0.5 wt % NH₄F.  (Line 216-218)

Q6: Figure 6, line 207: what was the content of fluoride (no data in a caption).

Authors’ response: The content of NH₄F used was 0.5 wt.%. The information is added into the caption of Figure 6 at page 8/20 as follows:

Figure 6. TNT features formed for 1, 15, 30, 60, 90, 120, 180, 360, and 720 min.  All anodization was done at 20 V in glycerol + water + 0.5 wt % NH4F.   (Line 229-230)

Q7: Line 135: There is also a minimum voltage at which the TNT structure forms; please give a reference and explain why it is so.

Authors’ response: Explanation describing the threshold voltage for TNTs formation has been added into the manuscript alongside a relevant citation as below.

Nevertheless, there is a limit to the voltage applied, as a high voltage may induce a too rapid electric field dissolution destroying the nanotubular structure. As reported by Lockman et al., depending on the electrolyte used, there is a threshold voltage at which the TNT structure is formed; TNTs are formed above the threshold voltage while a lower voltage would result in insufficient electric field dissolution generating TNTs with small pores [20]. (Line 141-147)

Q8: In the experimental part, there are no characteristics of the used power supply. Besides, please give the full characteristics of all purchased chemicals, such as the chemical purity, delivering company, and country, and for the equipment not only the name. But also the company and country.

Authors’ response: Thank you for pointing this out. All the details of the equipment and chemicals used are added into Section3:  Materials and Methods at Section 3 (Page 15/20) as below.

Ti foils (99.96% pure; thickness: 0.13 mm; Stream Chemical, USA) were cut into square sections (10  10 mm) for the experiment. Before anodization, the foils were degreased in an ultrasonic bath of acetone (J.T. Baker—9254, USA) and then ethanol (95.7% pure; Samchen, Malaysia) for 15 min each. The foils were then rinsed with deionized water and dried. The cleaned foils were then placed in an electrochemical cell with a restricted area of 5  10 mm exposed to the electrolyte. The anodization experiment was conducted using a two-electrode electrochemical cell at room temperature with a platinum cathode (diameter of 2 mm, 75 mm in length, Metrohm, Switzerland) and Ti foil as the anode using a DC power source (Agilent E3647A, USA). The distance between the anode and cathode was 30 mm. The electrolyte was a glycerol–water (85% and 15%, respectively) solution (Merck, Germany). Ammonium fluoride (NH₄F,Merck, Germany)) was added to the electrolyte in different amounts: 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 wt.%. To examine the anodization time effect, a 0.5 wt.% NH₄F electrolyte bath was used. The anodization time varied from 1 to 720 min at a constant voltage of 20 V. The effect of the anodization voltage was examined by varying the voltage from 1 to 80 V for 90 min in the same electrolyte bath (fixed sweep rate of 0.1 V/s).

The anodized Ti foil was removed from the electrolyte and rinsed with deionized water and dried in air naturally. The anodized Ti was then annealed using a horizontal tube furnace (Lenton 1200) at 450 °C for 3 h in air or nitrogen. The morphologies of the anodized foils were observed by field emission scanning electron microscopy (FESEM; Variable Pressure Zeiss Supra 35). Transmission electron microscopy (TEM; JEOL, JEM-2100F), at an acceleration voltage of 200 kV, was used to provide detailed observations of nanotube structure. X-ray diffraction (XRD; Bruker D8, Bruker GmBH, Germany) and Raman spectrometer (RENISHAW inVia 9P1567, UK) were conducted to determine the TNT phase and crystal analysis. UV–visible (UV–Vis) spectrophotometers were used to attain the diffuse reflectance spectra (Cary 5000) and conduct the Cr(VI) photoreduction (Perkin Elmer, USA).

For the Cr(VI) removal evaluation, 100 ppm Cr(VI) was prepared by dissolving 0.0283 g of potassium dichromate salt (K₂Cr₂O₇, A.R. grade, Merck, Germany) in 100 ml of deionized water. Then, the Cr(VI) stock solution was diluted to obtain the 5–20 ppm Cr(VI) solution. The pH of the solution was lowered using hydrogen chloride (Merck, Germany). For the hole scavenger, 1 mM of EDTA (Ethylenediaminetetraacetic acid, AJAX Chemicals, Sydney Australia) was added to the Cr(VI) solution. During the test, four anodized foils with a surface area of 1  0.5 cm² were immersed in the Cr(VI) solution. Before light irradiation, the solution was left in the dark for 30 min to achieve adsorption–desorption equilibrium. The solution was then exposed to sunlight with an average light intensity of ~ 1,000 Wm⁻², which was measured with a solar power meter (TM-207, Tenmars, Taiwan). During light irradiation, 2 ml of the aliquot sample was withdrawn every 15 min. The reduction of Cr(VI) was determined using 1,5-diphenylcarbazide (Merck, Germany), and, based on the UV–Vis results, the Cr(VI) concentration in the solution was calculated using Equation (14) at λ_max = 540 nm:  (Line 412-452).

Q9: The conclusions are limited only to summarizing the results and not to their scientific explanation. They should be rearranged to show the relationships and justify them.

Authors’ response: We agree with the reviewer comment. The conclusion was revised to summarize the results and findings from this study as follows:

In this study, removal and photoreduction of Cr(VI) solution using anodized TNTs under natural sunlight irradiation is demonstrated. The TNTs were formed by anodic oxidation of Ti foils in a fluorinated glycerol–water electrolyte. The effect of NH₄F concentration in the electrolyte, anodization voltage, anodization duration and annealing atmosphere (in air or nitrogen) on the nanotubes were systematically described. The minimum voltage for TNTs formation was identified as 5 V. To obtain TNT arrays with a clear, open top structure exhibiting a considerable length, anodization must be conducted using an electrolyte with an NH4F concentration of > 0.5 wt.% for more than 30 min. Higher NH4F content led to rigorous etching forming irregular TNTs. The diameter of TNT formed increased with voltage between the range of 10 to 60V while anodization at 80 V resulted in the collapse of the tubular structure. TNTs with an average length of ~2 µm were obtained after anodization for 180 min using an applied voltage of 20 V. From the XRD results annealed sample in air and nitrogen, the anatase crystallite size obtained was 36.79 and 26.29 nm, respectively. The smaller crystallite size of nitrogen-annealed TNTs is consistent with the Raman results obtained. The bandgaps of both annealed samples were determined using UV-Vis DRS indicating a smaller band gap for the nitrogen-annealed TNT compared to the air-annealed ones. This enabled an enhanced Cr(VI) photoreduction efficiency in the visible light region especially under natural sunlight irradiation. This is further demonstrated by the reduction of 10 ppm Cr(VI) solution that achieved a complete (100%) removal after 120 min of exposure to natural sunlight with the addition of EDTA as hole scavenger. (Line 457-477)

Q10: The change of Paste Tense to the Present Perfect is advised for sentences shown by color in lines 106-108, 110-112, 214-216, 228-229.

Authors’ response: As suggested by the reviewer, the sentences have been revised to present tenses in the manuscript as follows:

Figure 2 summarizes the findings, from which it is evident that, apart from the 0.1 wt.% NH₄F sample, a nanotubular structure is successfully form with the length and diameter affect by loading fluoride ions. (Line 107-109)

 However, higher concentrations of NH₄F (> 0.7 wt.%) reduce the length, which can be attributed to severe chemical dissolution at the TNT surface due to the increase number of fluoride ions. (Line 111-113)

 This shift  attributed to lattice distortion [41], perhaps induce by substitution of nitrogen within the crystal lattice, which result in strain [42–44]. (Line 238-240)

 The optical properties of the annealed TNT films are determined using diffuse reflectance UV–Vis spectroscopy, the results of which are shown in Figure 9. (Line 275-276)

Some sentences or phrases are unclear or inappropriate:

Q11: Line 225: The average crystallite size is 26.29 and 36.79 nm; rather `crystallite sizes were …`

Authors’ response: Thank you for the detailed checking. Revision has been made according to the comment by the reviewer.

The crystallite sizes were 26.29 and 36.79 nm for nitrogen- and air-annealed TNTs, respectively. (Line 249-250)

Q12: Line 64: fluoride ions have been identified as the most important parameter; rather `a presence of …`

Authors’ response: Revision has been made according to the comment by the reviewer.

For nanotubular formation, a presence of fluoride ions has been identified as the most important parameter. (Line 64-65)

Q13.: Line 122: oxide follows a flow mechanism in pore formation; unclear.

Authors’ response: The sentence described by the reviewer is revised to “The bottom part of the nanotubes, as seen in the cross-sectional images, is scallop-shaped, indicating a flow mechanism in the pore formation.” A detailed description of this mechanism is given in the subsequent sentences, at Line 123-128.

“The bottom part of the nanotubes, as seen in the cross-sectional images, is scallop-shaped, indicating that oxide follows a flow mechanism in the oxide pore formation. According to this mechanism, FRL will eventually be displaced as inter-pore materials. Pores are separated when the FRL is dissolved by water, resulting in discrete nanotubes. At larger amounts of NH4F, a thicker FRL is thought to form around the pores, and, as electrolyte is excess in water, the dissolution of this FRL will result in TNTs with very thin walls.” (Line 123-128)

Q14: Line 135: electric field dissolution; rather `electric-field enhanced …`

Authors’ response: The authors believe the correct term is “electric field dissolution”. For better clarity, the sentence has been rephrased as follows: “Nevertheless, there is a limit to the voltage applied, as a high voltage may induce a too rapid electric field dissolution destroying the nanotubular structure.” Line 140-142.

Q15: Line 164-166: at this voltage, the anodic oxide underwent severe polarization, which led to dissolution and the formation of an irregular porous structure.; anodic polarization, I presume? Dissolution of what? What does it mean, a flow mechanism in pore formation, flow of what? Be more precise.

Authors’ response: Thank you for the comment. For better clarity, the sentence has been rephrased to precisely describe the collapse of the nanotube structure at high voltage anodization as follows: “At 80 V, the nanotubular structure is destroyed (Figure 3 (j)) due to the overaccelerated electric field dissolution causing severe polarization and weakening of the Ti-O bond, which led to the formation of irregular porous oxide structure.” (Line 177-181)

Q16: Line 234: The…function multiplied by … using a corresponding coefficient (n); unclear.

Authors’ response: From the description Kubelka Munk given in Line 276-279, "the corresponding coefficient (n) is the value in (F(R) )ⁿ equation (represent as 0.5 or 2 based on direct or indirect bandgap metal oxides)". The description is also newly added into the manuscript.

Q17: Line 235: The n values of 0.5 and 2 used to estimate direct and indirect bandgap oxide; rather `…were used…`

Authors’ response: Thank you very much for the correction. Revision has been made according to the reviewer’s suggestion as follows:

The n values of  and 2 were used to estimate direct and indirect bandgap oxide, respectively. (Line 283-284)

Q18: Line 294-295: Reduction experiment (Figure 11) shows reduction was very fast; suggested `The experiment showed very fast reduction` (of what?)

Authors’ response: Thank you for the correction. The sentence has been revised as follows:

The experiment (Figure 12) showed very fast reduction of 5 ppm Cr(VI) solution with 100 % reduction after 90 min of sunlight exposure. (Line 358-359)

Q19: Line 329: 111Table 2; delete 111.

Authors’ response: As pinpointed by the reviewer, the number 111 has been deleted.

Table 2 compares the Cr(VI) reduction efficiency obtained in the present paper with existing literature, from which it is evident that the photocatalytic reduction of nitrogen-annealed TNTs is comparable with fluorine- and carbon-doped TNTs under UV–Vis light irradiation [45,61–65]. (Line 393-396)

Q20: Lines 332-333: adding a small amount of EDTA (1 mM) amounts under UV–Vis light irradiation, such as phenol (10 mM) and tartaric acid (6 mM) …; unclear.

Authors’ response: We would like to thank the reviewer for comment. The authors wanted to describe that a lower amount of EDTA addition led to a higher Cr(VI) photoreduction efficiency compared to addition of other scavengers at a higher amount. Therefore, the sentence has been revised as follows:

Furthermore, by adding a small amount of EDTA (1 mM), a higher efficiency of Cr(VI) photoreduction was obtained compared to addition of  other scavengers such as phenol (10 mM) and tartaric acid (6 mM) at higher amounts under UV–Vis light irradiation. (Line 396-399)

Q21: Line 357: The anodized Ti foil was removed from the electrolyte and cleaned; how was it cleaned?

Authors’ response: The anodized Ti foils were cleaned by rinsing in DI water and dried in air. The description is also newly added into the revised manuscript as follows:

The anodized Ti foil was removed from the electrolyte and rinsed with deionized water and dried in air naturally. (Line 428-429)

Q22: Line 383: proposed electrolyte is dependent on NH4F concentration, anodization duration, and applied voltage; both unclear and improper.

Authors’ response: Thank you for the comment. The statement has been rewritten to indicate a clearer statement of the parameters investigated in this study as follows:

The effect of NH4F concentration in the electrolyte, anodization voltage, anodization duration and annealing atmosphere (in air or nitrogen) on the nanotubes were systematically described. (Line 459-461).

Q23: Line 144, 146: different loadings of NH4F; rather different contents or concentrations

Authors’ response: Thank you very much for the suggestion. The pinpointed sentences have been revised accordingly in the revised manuscript as below.

Figure 1. FESEM of anodised TiO₂ at 20 V for 90 min in the glycerol–water electrolyte with different concentrations of NH₄F: (a) 0.1 wt.%; (b) 0.3 wt.%; (c) 0. 5 wt.%; (d) 0.7 wt.%; (e) 0.9 wt.% and (f) 1.0 wt.%. (Line 155-156)

 Figure 2. Anodic TiO₂ formed at 20 V for 90 min in the glycerol–water electrolyte with different concentrations of NH₄F: 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 wt.%. (Line 157-158)

Reviewer 2 Report

There is a lot of information in the literature on the use of TiO2 nanotubes to degrade various pollutants. Based on the information presented in the manuscript, the reviewer must necessarily conclude that the scientific novelty of this work is negligible. The authors present results that are well described in the literature namely TiO2 morphology formed in the electrolyte composed of glycerol. Moreover, the presented physicochemical characteristics of the obtained materials are insufficient, there is no XPS, Raman, FTIR, XRD analysis for all the samples obtained.

After clearly indicating the novelty of the work and supplementing the gaps in the characterization of obtained materials, the reviewer's decision may be changed.

Author Response

Authors’ response:

We would like to thank the reviewer for the comments.

1. TiO₂ is a wide band gap semiconductor which can only be activated under ultraviolet (UV) illumination.  In here, post annealing in nitrogen was done as to induce defects within the oxide which can narrow down the band gap. The use of natural sunlight is obviously more robust than the use of UV lamp for catalyst activation. 

There are many works done on the degradation of organic contaminants such as dyes but not on the removal of heavy metal ions such as Cr(VI) via catalytic process on TiO₂ nanotubes.  Moreover, it can be seen that the nitrogen annealed nanotubes can remove 10 ppm Cr(VI) solution within 2 h under sunlight irradiation which can be considered rather fast. Kinetics studies on the reduction are also presented.  

Photocatalytic activity of TiO₂ is mostly reported and studied in its powdered form. However, powdered photocatalysts have serious drawbacks, such as problems with separation and reusability which extremely limits the application.  TiO₂ nanotubes on the other hand were grown on titanium and hence can be considered as supported catalysts.  The catalysts can be reused to perform reduction process many times. 

Indeed, there are various methods for fabricating TiO₂ nanotubes but anodization has been seen as a rather simple technique for producing supported oxide catalysts.  Organic electrolyte has been routinely used for TiO₂ nanotubes formation with smooth walls, focusing mostly on the use of ethylene glycol with volume % of water of less than 5 %.  However, in this work, glycerol with excess water (15 %) was chosen and as to date not a lot of work has been done on such electrolyte composition. The use of this electrolyte produced nanotubes with highly serrated walls (which could be good for increased in surface area) and rather big diameter nanotubes and hence both the exterior and interior of the nanotubes can be used.  

The abovementioned statements indicate that this study on the removal and photoreduction of heavy metal Cr(VI) ions using TNTs obtained by anodization in glycerol-water electrolyte under natural sunlight, possesses good novelties that warrant its publication. We hope these satisfy the reviewer's query regarding the novelty. 

2. In regards to the characterization supplement, the XRD analysis for the as-anodised, air-annealed, and nitrogen-annealed TiO₂ nanotubes is presented in Figure 7 . Additional Raman results are also added as Figure 9 into the revised manuscript. The detailed description can be found below.

 XRD results: Please refer Figure 7 for XRD analysis for TNT_as-anodised, TNT_air and TNT_nitrogen (Subsection 2.1.4, Page 8, Line 233-252)

Description in the manuscript:  Samples prepared by anodization at 20 V were annealed either in air or nitrogen at 450°C for 3 h. The XRD pattern in Figure 7 (a) shows that the as-anodized TNTs are amorphous, and, after annealing, anatase TiO₂ (ICSD: 98-010-7874) can be identified from peaks at 25.4°_, 48.1°, and 54.7°, corresponding to the (011), (020), and (121) planes, respectively. Minimal differences can be observed for samples annealed in air and nitrogen, apart from a slight shift in the (011) peak, as shown in Figure 7 (b). This shift  is attributed to lattice distortion [41], perhaps induce by substitution of nitrogen within the crystal lattice causing strain [42–44]. Since the ionic radius of nitrogen ( = 1.46 Å) is larger than that of oxygen ( = 1.38 Å), the substitution of nitrogen in the O-site of the crystal structure may result in the expansion of TiO₂ crystal lattice due to tensile strain [42].

The crystallite size for air- and nitrogen-annealed TNTs was calculated using the Debye–Scherrer equation at the (011) anatase peak according to the full width at half maximum of the diffraction pattern. The crystallite sizes obtained were 26.29 and 36.79 nm for nitrogen- and air-annealed TNTs, respectively. The smaller crystallite size for the nitrogen-annealed TNTs can be attributed to growth suppression during annealing in nitrogen environment [45].

Raman results:  Please refer Figure 8 for Raman analysis for TNT_air and TNT_nitrogen (Page 9, Line 253-267).

Description in the manuscript: To further study the changes in phase structure after annealing in air and nitrogen environment, Raman spectroscopy on the TNTs was carried out and the results are shown in Figure 8. When the TNTs were annealed at 450 °C, both samples exhibited three typical modes corresponding to A_1g (515 cm^-1), B_1g (398 and 515 cm^-1) and E_g (144, 197, and 640 cm^-1), respectively in the Raman spectrum [39]. The peaks in these modes demonstrates the presence of anatase phase as predominant crystal structure. The anatase peaks of TNT are labelled with A (Figure 8 (a)). The intensity of the dominant anatase peak at 144 cm^-1 is higher for the TNTs annealed in nitrogen compared to air. Moreover, from the enlarged image of image of the dominant peak in Figure 8 (b), it can be observed that the bandwidth increased for the nitrogen-annealed sample. These results indicate that the crystallinity of the anatase phase was enhanced and the crystallite size decreased after annealed in nitrogen, which is consistent with the results of the XRD measurement.

Round 2

Reviewer 1 Report

Thank you for the exhaustive answers and proper corrections. I have no other remarks.

Reviewer 2 Report

The manuscript can be accepted as is. Most of the deficiencies have been corrected