Review Reports - Tailoring Black TiO2 Thin Films: Insights from Hollow Cathode Hydrogen Plasma Treatment Duration

Round 1

Reviewer 1 Report

All things considered, I am pleased to provide a positive review of the scientific paper. After thoroughly reading the research presented, I can say that I have no objections to the findings. The paper is well-rounded, clear, and showcases thorough investigation on the topic. The authors have skillfully presented their research in a logical and coherent manner, making it easy to follow and understand.

Of course: (i) up to date references can be added to introduction part and also (ii) little bit more about the idea behind this study can be added.

Minor editing of English language required

Author Response

Of course: (i) up to date references can be added to introduction part and also (ii) little bit more about the idea behind this study can be added.

Response: Thank you for your constructive feedback on our scientific paper. We take your suggestions and agree that adding up-to-date references to the introduction will further strengthen the paper. We reviewed the latest literature and incorporated some relevant references to provide a more comprehensive background. Additionally, we understand your point about elaborating on the idea behind the study. We revised the introduction to provide more context and clarify the motivation and rationale behind our research.

Reviewer 2 Report

The research provided is a continuation of the work first described in https://doi.org/10.3390/catal10030282 . It provides valuable insights into the role of hydrogenation time using the HCHP treatment route on anatase TiO2 thin films and demonstrates the potential of the produced black TiO2 thin films photocatalytic applications.

The manuscript is well written, easy to understand and to follow. Proper State-of-Art of TiO2 modification approaches with the aim to narrow band gap and increase maximum absorption of radiation in the visible region were well prepared and presented in the Introduction part. All the experimental activities were described in sufficient detail. However, some improvements still could be considered before publishing. In particular:

1. How photodegradation rates were comparable with the results received by other authors using MB dye decomposition experiments with the black TiO2 prepared by other methods?

2. How was the temperature of the hydrogen plasma treatment measured?

3. The manuscript concentrates on the TiO2 photocatalytic applications. However, in the keywords and abstract photovoltaics and application in solar cells are mentioned but insufficiently shown in the experimental and results analysis part. I would suggest removing it and staying on photocatalytic applications only.

Author Response

The research provided is a continuation of the work first described in https://doi.org/10.3390/catal10030282. It provides valuable insights into the role of hydrogenation time using the HCHP treatment route on anatase TiO2 thin films and demonstrates the potential of the produced black TiO2 thin films photocatalytic applications.

How photodegradation rates were comparable with the results received by other authors using MB dye decomposition experiments with the black TiO2 prepared by other methods?

Response: It should be emphasized that while alternative methods reported in the literature, such as the sol-gel and gas-phase oxidation techniques, yield TiO2 with promising photocatalytic activity, a direct comparison with the black TiO2 thin films generated in the current study may not be entirely accurate. This discrepancy arises due to variations in preparation techniques, treatment durations, experimental conditions, and the specific forms of TiO2 produced - predominantly nanoparticles or nanostructures in the literature versus thin films in our work. Furthermore, within the constraints of the time allotted, I was unable to find direct comparisons of methylene blue photodegradation rates for black TiO2 prepared using these alternate methods. Additionally, this comparison will be focus of a future work of the research group.

How was the temperature of the hydrogen plasma treatment measured?

Response: As cited in Supplementary Material, reference [2]: “The hollow cathode wall temperature was measured by an optical pyrometer (Raytek Corporation, Raynger 3I, Santa Cruz, CA, USA), reaching a maximum value of 260 ºC after 8 min and maintaining until the end of the treatment.”

The manuscript concentrates on the TiO2 photocatalytic applications. However, in the keywords and abstract photovoltaics and application in solar cells are mentioned but insufficiently shown in the experimental and results analysis part. I would suggest removing it and staying on photocatalytic applications only.

Response: We concur with the reviewer's suggestion and have accordingly removed references to photovoltaics and solar cells from the keywords, abstract and conclusion. This adjustment ensures consistency with the primary content of the paper.

Reviewer 3 Report

This paper reports HCHP treatment on TiO2. The TiO2 films are characterized. The results are suitable for future researchers’ reference. Therefore, I recommend that this paper can be documented in Plasma journal.

1. One interesting point is the stability of the film. How stable is the HCHP treated TiO2? Will it degrade with time in typical storage environment?

2. Figure 8 and Figure 9. How reliable is this method to obtain the tail state. It seems that the fitting of XPS tangential line is not very reliable.

3. Figure 11. What’s the basic mechanism for improved MB photodegradation with HCHP-treated TiO2.

Author Response

One interesting point is the stability of the film. How stable is the HCHP treated TiO2? Will it degrade with time in typical storage environment?

Response: Thank you for your valuable comments and the recommendation to publish our work in the Plasma journal. We appreciate your interest in the stability of the HCHP (High-temperature Continuous Hydrothermal Process) treated TiO2 films.

As per your query regarding the stability of HCHP treated TiO2, we have conducted preliminary tests which indicate that these films exhibit good stability under typical storage conditions. However, we acknowledge that a more comprehensive study involving long-term stability analysis under different environmental conditions would be needed for a complete assessment. We plan to address this in our future work.

Figure 8 and Figure 9. How reliable is this method to obtain the tail state. It seems that the fitting of XPS tangential line is not very reliable.

Response: Thank you for your question regarding the reliability of the method used to determine the tail states using XPS and the fitting of the tangential line. The tail states were discerned based on a model that considered the presence of Ti3+/2+ defect band states and the expansion of tail states due to increased treatment time in the HCHP process. We recognize that interpreting XPS spectra, especially during the deconvolution process, can be intricate and may involve a certain degree of subjectivity. However, we've made every effort to ensure that our fitting process is as accurate as possible. As for your concern about the fitting of the tangential line, it's important to note that this fitting was performed using the linear fitting feature of Origin software in the region that appeared most linear. This comprehensive approach helps mitigate potential errors and enhance the reliability of our results.

Figure 11. What’s the basic mechanism for improved MB photodegradation with HCHP-treated TiO2.

Response: The improved methylene blue (MB) photodegradation with HCHP-treated TiO2 can be attributed to several key mechanisms:

Interstitials and Oxygen Vacancies: The concentration of Ti3+/2+ interstitials and oxygen vacancies (Vo) on the film surface significantly enhances the photocatalytic activity of the HCHP-treated TiO2. These elements contribute to the creation of defect states within the bandgap of the material, which can trap charge carriers and prevent their recombination, thereby enhancing the photocatalytic efficiency.
Hydroxyl Radicals: The formation of hydroxyl radicals (OH) on the film surface also plays a critical role. These radicals are known to be highly reactive and are primarily responsible for the oxidative degradation of organic pollutants like MB.
Increased Surface Area: HCHP treatment also increases the surface area of the TiO2 films, which can contribute to improved photoactivity. This is because a larger surface area can accommodate more photocatalytic active sites, thereby enhancing the degradation of dye.

However, it's worth noting that while HCHP treatment generally improves photocatalytic activity, it doesn't lead to a consistent increase with longer durations of treatment. Prolonged hydrogenation can generate a high concentration of oxygen vacancies (Vo) within the TiO2 bulk, which may serve as recombination centers for electron-hole pairs, potentially reducing the efficiency of photocatalytic degradation. This might explain why the samples treated for 15 and 30 minutes (B-15 and B-30) showed better photodegradation performance than those treated for longer durations (B-45 and B-60).

Reviewer 4 Report

I read the text of the article with great interest. I think that the issue taken up is very interesting. The exemplary discussion of the results attracted my attention. In my opinion, the article could be accepted for publication as it stands, although I would like to point out to the authors some editorial remarks worth considering before publishing it. I provide the comments in the form of the following list.

1. Line 142

Was XPS examination preceded by sputtering processes on the surface zone of the samples?

2. Table 1

The text misses any mention of the methodology for determining surface energy. Not every article reader will know that surface energy is related to the contact angle.

3. Line 188 – the "grains" term

The term "grain" in materials science refers to a single crystallite. Grain and crystallite are synonyms. I think it is a mistake to call the forms visible on the surface of the layers by the term "grain." The visible particles are more significant than the calculated sizes of crystallites/grains in the XRD method. Instead, the globular forms are agglomerates of particles formed in the plasma, also supplied with mass by the adhesion of atomic dispersion particles. I suggest the authors use a different term, such as particles/ agglomerates.

4. Figure 1

AFM images are characterized by inferior quality. In the printed version, seeing details such as particle sizes and values on the axes is difficult.

5. Figure 2

It seems that this figure is not necessary. Surface area values can be presented in Figure 1.

6. Figures 2 and 10

It seems that linking the measured data on the graphs by straight line segments is incorrect from the point of view of the methodology of presenting results. If the authors want the data to be combined, making a fit with some curve would be necessary. I think it is safer to present such results using bar charts.

7. Figure 10

There is a unit missing in the description of the Y axis

8. Variables

Variables in the text should be italicized

Author Response

REVIEWER #4

Line 142

Was XPS examination preceded by sputtering processes on the surface zone of the samples?

Response: Thank you for your valuable feedback and positive evaluation of our work. We appreciate your careful reading of the manuscript and your thoughtful comments.

To answer your question regarding line 142: The XPS examination was not preceded by any sputtering processes on the surface zone of the samples. We chose not to conduct sputtering to avoid any potential alterations to the surface chemistry of the samples which could potentially influence our XPS results.

Table 1

The text misses any mention of the methodology for determining surface energy. Not every article reader will know that surface energy is related to the contact angle.

Response: We appreciate your insightful question. You are correct in noting that we did not include a clear explanation of the methodology used to determine the surface energy. We apologize for the oversight and agree that such information would be beneficial to readers:

“Surface energy values were calculated with a software program (DROPimage Advanced, version 2.4) that uses the harmonic mean method applied to the acid-base theory.”

Line 188 – the "grains" term

Response: We appreciate the reviewer's feedback concerning the use of the term "grain" in our manuscript. We understand that in the field of materials science, "grain" typically refers to a single crystallite. Given the size of the forms visible on the surface of the layers in our study, we agree that there may be some confusion regarding the use of this term.

In light of this feedback, we agree that using a term such as "agglomerates" might provide a more accurate description of these structures. This change in terminology could also help clarify the nature of the structures observed on the material surface.

Figure 1

AFM images are characterized by inferior quality. In the printed version, seeing details such as particle sizes and values on the axes is difficult.

Response: New higher resolution images have been added in Figure 1.

Figure 2

It seems that this figure is not necessary. Surface area values can be presented in Figure 1.

Response: Surface area values have been added to figure 1. Figure 2 has been removed.

Figures 2 and 10

Response: Figure 10 (now Figure 9) has been reworked per the reviewer's recommendation.

Figure 10

There is a unit missing in the description of the Y axis.

Response: The unit of sheet resistance is given in kilo-ohms per square. Usually, the square unit is represented by □. We put a comment in the figure caption to clarify this doubt.

Variables

Variables in the text should be italicized.

Response: We correct the variables in the reviewed article.

Reviewer 5 Report

In this work, the authors report the hydrogenation of anatase TiO2 thin films using a radio frequency plasma-assisted chemical vapor deposition (RF-CVD) device for enhancing light-absorbing and electrical properties. Also, the authors systematically analyzed the properties of TiO2 with HCHP treatments. However, the reviewer thought that some issues should be resolved to get broad interest in the journal of Plasma.

1) The reviewer highly recommends all authors rephrase their works with the newer, novel, and highly impacted references to publish this manuscript. Especially, the introduction part should be improved for shed light.

2) In Figure 5, the Raman shift at 140 cm-1 was observed. What is the reason for this tendency?

3) The authors need to present the mechanism for the change of TiO2 properties according to HCHP treatment in detail.

4) Line 241 should be corrected. It may be 3.3.

English style is fine.

Author Response

Response: Regarding the introduction part, we agree with the reviewer's advice to improve it. We revised this section to better highlight the novelty and importance of our research.

2) In Figure 5, the Raman shift at 140 cm^-1 was observed. What is the reason for this tendency?

Response: Both blue-shift and FWHM increase of the main E_g vibrational mode detected after TiO₂ hydrogenation was also observed in previous works and is attributed to the structural changes that the TiO₂ suffers by the induced lattice defects, i.e., structural disorder promoted by the creation of V_O (https://doi.org/10.1016/j.jcis.2020.01.079, https://doi.org/10.1021/ja3012676). This answer has been added to the manuscript.

3) The authors need to present the mechanism for the change of TiO2 properties according to HCHP treatment in detail.

Response: The HCHP process is used to hydrogenate titanium dioxide (TiO2) thin films, leading to a transformed variant, black TiO2. The process primarily induces oxygen vacancies (Vo) and promotes the reduction of Ti4+ oxidation states to Ti3+/2+ species. These changes result from the interaction between reactive hydrogen species and the TiO2 thin films, leading to a series of chemical reactions that result in the formation of Ti2O3 and TiO species, along with H2O volatile molecules. The hydrogenation process thus decreases electron density around Ti atoms and induces a shift in the Ti 2p3/2 peak. The HCHP process also expands tail states between the valence and conduction bands, increasing the defect density of Ti3+/2+. These modifications enhance the electrical conductivity and optical properties of TiO2, which are crucial for its practical applications. This discussion is presented along the results and discussion text and also we highlighted in conclusion topic.

4) Line 241 should be corrected. It may be 3.3.

Response: Corrected.