Ti-Mo-O Nanotube Arrays Grown by Anodization of Magnetron Sputtered Films

Round 1

Reviewer 1 Report

This article describes very interesting theme, and obtained results look promising. On the other hand, the introduction is not clear what is the purpose of the research, and due to unclear purpose, the obtained results’ significance is vague. It is highly suggested to reconstruct the structure, specifically a bit clearer relationship between the purpose and the obtained results.

Some moderate level of English description reconsideration would be highly recommended to clarify the specific research point. In addition to English, references are not consistent in their description, moreover, some important information such as year of publish is missing at some citations.

Author Response

Thank you for your comment.

The main idea presented in this work is the development of a method for directly fabricating nanotubes from solid state solution precursors. Thermal treatment, for example, will allow the preparation of nanoarrays for photocatalysis or sensor devices.

We have added a few sentences to the relevant sections (Abstract, Introduction, and Conclusion) that clarify our efforts. Sentences are highlighted in yellow in the manuscript.

For example, at the end of introduction we added:

We decide not to modify grown TiO₂ nanotube, but directly prepare alloyed nanotubes fore solid-state solution precursor. It is possible to expect changing of bandgap value due to significant presence Mo in the lattice in compare to TiO₂. This will lead to more efficient utilization of solar light and improvement of photocatalytic and antimicrobic properties [31].

Reviewer 2 Report

In the present work, the authors report the use of new method coupling the magnetron spluttering and electrochemical oxidation to generate Ti1-xMoxO2 alloyed materials with high Mo content. The approach is of interest but the presentation of the results needs to be improved with additional data. I could consider for publication but with major revision. 

1/ During the anodization, it is mentioned that the voltage is fixed at 40 V but then the authors continue by "to avoid rapid dissolution of nanotubes, the current was kept constant at 3 µA/cm2. How it is possible to simultaneously maintain the potential and the current at fixed values ? 

2/ The authors claim that the applied bias during the deposition strongly affects the morphology and structure of the resulted TiMo film. However, only the characterizations of the biased substrate are presented. Accordingly, it is impossible for me to agree to your comment. 

3/ Could you comment on why the different composition of the precursor (Ti/Mo = 0.9 (B) and 0.6 (C)) conducts to the same Ti/Mo ratio of the product (around 1)

3/ There is a big confusion for me. As the authors provide the SEM, EDS of the sample B but the table 2 is about the sample C. Also, the authors claim that the composition from STEM - EDS (table 2) is similar to SEM-EDS (table 1). However, there is a very strong discrepancy in the result: 16.4 % Ti/24.2 % Mo (Table 2) versus 17.2 % Ti/ 16.5 % Mo. Could you provide an explanation to the observation ? 

4/ The influence of the composition in precursor is given. However, all the parameters concerning the deposition process is not detailed, including the influence of the deposition potential, deposition time ? Do you investigate those parameters? 

5/ The average length and diameter of the nanotubes are given without any standard deviation values. Could you include them ? 

6/ There is the image E that is overlapped with other images and there is no caption related to this image. Furthermore, there is the word A1 on the figure D1

 

 

Author Response

Thank You for your valuable comments, which make the manuscript much clearer. First, we would like to mention that our manuscript mainly focused on the method of nanotube fabrication, which is why we have not performed a comprehensive study of all possible analyses yet. In-depth research requires significantly more effort and time, so we plan to publish it later.

1/ During the anodization, it is mentioned that the voltage is fixed at 40 V but then the authors continue by "to avoid rapid dissolution of nanotubes, the current was kept constant at 3 µA/cm2. How it is possible to simultaneously maintain the potential and the current at fixed values ? 

Thank you for this comment. Part 2.2.” Anodic oxidation”, did not describe the oxidation process clearly. It was rewritten.

 

2/ The authors claim that the applied bias during the deposition strongly affects the morphology and structure of the resulted TiMo film. However, only the characterizations of the biased substrate are presented. Accordingly, it is impossible for me to agree to your comment. 

In the presented results we do not have direct evidence the structure changes due to the bias. Therefore, we reformulated the text in the following form:

An indication of the difference in structure is shown in fig. 3, where the layers without bi-asing show a sharper boundary between individual columnar grains compared to those with applied bias. Moreover, in the following text we show that on precursor layers pre-pared with bias we were able to prepare nanotubes, which provides additional indirect evidence for nanopores suppression.

3/ Could you comment on why the different composition of the precursor (Ti/Mo = 0.9 (B) and 0.6 (C)) conducts to the same Ti/Mo ratio of the product (around 1)

To our understanding, the decreasing of Mo content in nanotubes is related to forming of volatile MoO₃ oxide during anodic oxidation. For sample with higher molybdenum percentage, the trend is even more visible for example in sample D Ti/Mo ratio increases from 0.4 to 0.6.

3/ There is a big confusion for me. As the authors provide the SEM, EDS of the sample B but the table 2 is about the sample C. Also, the authors claim that the composition from STEM - EDS (table 2) is similar to SEM-EDS (table 1). However, there is a very strong discrepancy in the result: 16.4 % Ti/24.2 % Mo (Table 2) versus 17.2 % Ti/ 16.5 % Mo. Could you provide an explanation to the observation ? 

All the samples were analyzed by top-view SEM-EDS and results are presented in Table 1.

We agree that there is some discrepancy between the absolute values of element content. It is given by the possibilities and principles of both methods. Lateral resolution of SEM EDS is about 0.5 µm³ (depending on electron beam parameters and sample material and structure). Typically, we analyze the area at 100x100 µm.  To the contrary, the volume analysed by STEM - EDS is tens of cubic nanometers.  =+That is why there are possible some variation in STEM -EDS measurements.

In our manuscript, the main purpose of STEM EDS was to show uniformity of composition across the nanotube.

To avoid any misinterpretation, we decided not to present quantitative STEM EDS results.

 4/ The influence of the composition in precursor is given. However, all the parameters concerning the deposition process is not detailed, including the influence of the deposition potential, deposition time ? Do you investigate those parameters? 

Yes, we investigated more parameters than were presented. The deposition voltage was from 550 to 1000 V. However, initial results had not shown any significant improvement in the oxidation process, and for this reason we abandoned their further investigation and fixed the voltage at 650 V (The current was typically in the range 450-500 mA). Deposition time influences mainly the thickness of the film. For our purpose, thickness about 700 nm films was sufficient; in our system, this corresponded to 8 minutes of deposition. Overall, the main aim of our article is to present a method for Ti/Mo nanotube fabrication but not a sophisticated investigation deposition process.

 

5/ The average length and diameter of the nanotubes are given without any standard deviation values. Could you include them ? 

Thank you for your comment. These values have been included into the text.

6/ There is the image E that is overlapped with other images and there is no caption related to this image. Furthermore, there is the word A1 on the figure D1

The image E overlaps other images by purpose. Unfortunately, we have not found another way present the sample E. On the sample E the precursor layer was over-etched by electrolyte, even the silicone substrate was attacked by fluorine. In this case, no nanotubes were formed.

However, we decided to remove image of sample E from the manuscript, because it does not provide any valuable information.

A1 has been removed from the image D1. It was a mistake.

Reviewer 3 Report

Dear Authors,

Your paper on growing arrays of Ti-Mo-O nanotubes is interesting due to its experimental results. In my opinion, in order to improve the manuscript, it is desirable to address the following comments and questions:

1) clearly define the purpose of your work both in Abstract and in the end of Introduction;

2) in Abstract, indicate the novelty of your work;

3) in Conclusion, indicate the novelty of your work, taking into account the results obtained by other authors;

4) please explain in the end of Introduction the reason for your interest in the Ti-Mo-O system and the choice of the Mo concentration range from 32 up to 82 at %. As follows from other publications (e.g. Choi, W., Termin, A., Hoffmann, M. R. doi:10.1021/j100102a038) iron doping from 0.1 up to 2.0 at % gives a better result than Mo.

 

 

Author Response

Dear Referee, thank You for the comments.

We have added a few sentences to the relevant sections (Abstract, Introduction, and Conclusion) that clarify our efforts and the novelty obtained in our work. Sentences are highlighted in yellow in the manuscript.

In the abstract we underlined that we growth nanotubes directly from solid solution of TiMo, to  a contrary to most publications where TiO₂ tubes doped after growth.  Also we mentioned tha we obtained homogenous Ti-Mo-O composition across the nanotube.

In particular we added following text to the end of introduction:

We decide not to modify grown TiO2 nanotube, but directly prepare alloyed nanotubes fore solid-state solution precursor. It is possible to expect changing of bandgap value due to significant presence Mo in the lattice in compare to TiO2. This will lead to more efficient utilization of solar light and improvement of photocatalytic and antimicrobic properties [31].

Similar, but shorter, information was added to conclusion.

Regarding forth comment: in our work, we also considered the technological aspect of preparing Ti-Mo-O thin films using magnetron sputtering. Iron, as a ferromagnetic material, is in principle not appropriate for the preparation of films by this technology.

According to your advice we added such explanation to the end of introduction:

We decide to use molybdenum instead of iron, which may perform better photoreactive results according to [30], because ferromagnetic properties of iron can significantly influence the geometry of the discharge and make deposition hardly repeatable.

Round 2

Reviewer 2 Report

After revision, the manuscript is in much better shape. I would recommend for publication in the present form