Black TiO2 Thin Films Production Using Hollow Cathode Hydrogen Plasma Treatment: Synthesis, Material Characteristics and Photocatalytic Activity
, André Pereira 2, Marcilene Gomes 3, Mariana Fraga 4, Rodrigo Pessoa 1, Douglas Leite 1, Gilberto Petraconi 1, Adailton Nogueira 5, Heberton Wender 5, Walter Miyakawa 6, Marcos Massi 7 and Argemiro da Silva Sobrinho 1,*Round 1
Reviewer 1 Report
In this paper, the authors synthesized black TiO2 thin films using hollow cathode hydrogen plasma treatment. Black TiO2 nanoparticles has been well studied by researchers for decade, including plasma treatments, in the present study, they investigated the synthesis of black TiO2 film, by using direct current magnetron spattered TiO2. While the superior point on this film method over nanoparticle ones is not clearly mentioned, they carefully examined the structural properties of synthesized film and characterized the photocatalytic activity. Therefore, I recommend to be published after minor revision as mentioned below.
HR-TEM. The reviewer recommend to add HR-TEM images for black TiO2 to clearly display the introduction of surface disorders. Figure 7. X-axis value scale is missing.Author Response
We would like to thank the reviewers for their comments / suggestions and we hope that this revised version of our manuscript – that takes into consideration the reviewers’ suggestions – meets the requirements for publication in the Catalysts Journal. Changes were made using Microsoft Word "track changes" feature, as recommended, and texts added to the manuscript are highlighted in yellow.
Best regards,
The authors
Response to the Reviewer's comments and list of changes made to the manuscript:
Reviewer 1
1. “HR-TEM. The reviewer recommend to add HR-TEM images for black TiO2 to clearly display the introduction of surface disorders.”
Response: We thank the reviewer for the recommendation and we agree that the HR-TEM is an analysis that can contribute to the discussion. However, unfortunately, there is no such equipment in the institutions of the authors and we do not have access to the institution that has this equipment, a fact that did not make it possible to add this analysis. We believe that, with the results and discussions presented in the paper by the analyses carried out, the issue of surface disorder is well exposed and explained. Even so, we thank the reviewer again for this suggestion and we will consider this analysis in our future works.
2. “Figure 7. X-axis value scale is missing.”
Response: We thank the reviewer for observing this issue. Figure 7 was corrected.
Reviewer 2 Report
This manuscript describes the production of black TiO2 thin films using hollow cathode hydrogen plasma treatment for photocatalyst. Overall speaking, the manuscript is interesting and well organized, which could be considered for publication after revisions in addressing the following comments.
In line 132, the author declares that the HCHP treatment is found to be responsible for the slight decrease in the thickness of the black TiO2 thin film. Why would the HCHP treatment cause the thickness decrease?
How about the cost of this method? Can we consider it for mass production?
Author Response
We would like to thank the reviewers for their comments / suggestions and we hope that this revised version of our manuscript – that takes into consideration the reviewers’ suggestions – meets the requirements for publication in the Catalysts Journal. Changes were made using Microsoft Word "track changes" feature, as recommended, and texts added to the manuscript are highlighted in yellow.
Best regards,
The authors
Response to the Reviewer's comments and list of changes made to the manuscript:
Reviewer 2
1. “In line 132, the author declares that the HCHP treatment is found to be responsible for the slight decrease in the thickness of the black TiO2 thin film. Why would the HCHP treatment cause the thickness decrease?”
Response: The hydrogen plasma generated during the HCHP treatment is very dense and energetic, which promotes a bombardment of H+ ions with high energies on the film surface. This bombardment can cause a chemical etching of the thin surface or even create OH volatiles species. Also, due to the high energy gained by hydrogen ions when passing through the cathodic plasma sheath (higher than 200 eV), some ions can be implanted at depths of several nm, promoting the disordering by formation of oxygen vacancies (Vo). The combination of these two phenomena can be responsible for the slight thickness decreasing, verified by the profilometry analysis. To better clarify this, we added this explanation to section 2.1 lines 144 to 154 (highlighted in yellow).
2. “How about the cost of this method? Can we consider it for mass production?”
Response: Regarding the costs of this method, considering a rough estimate of approximately 1700 W of power spent by the system (RF source + vacuum pump + other devices), the amount of energy consumed in 15 min would be around 0.425 kWh. The consumption of H2 gas is low, since the process is carried out with a gas flow rate of 45 sccm (or 0.045 L / min). Comparing with conventional processes for black TiO2 synthesis (ref. [8] of the paper), where annealing (temperature approx. 500 ºC) times of the order of days and high pressures of H2 (order of bars) are required. For an oven to reach a temperature of 500ºC, powers of the order or above 500 W are required, so, in a rough estimate, the power spent in a 1-hour and 24-hour processes would be in the order of 0.5 kWh and 12 kWh, respectively. Fact that would considerably increase the energy costs for mass production in a period of one month for the conventional method in comparison with plasma method.
Regarding the mass production of black TiO2 material using HCHP, there are currently commercial low-pressure plasma equipment that allows the mass production of thin films on wafers over 400 mm in diameter, a fact that has allowed to reduce the cost of a chip based on planar technology, for example. In these reactors, the biased electrode generally has a planar geometry, which facilitates the scaling of the process. For hollow cathode devices, in special those based in tubular geometry, it is possible to scale the electrode to larger volumes, following the scaling law called “White-Allis similarity law” for hollow cathodes (ref. [33] of the paper). However, in terms of scaling, a negative point in relation to reactors based on planar electrodes would be the considerable increase in the volume of the reactor, which would require greater applied power to generate the same plasma density, greater pumping capacity of the vacuum system, etc.
Reviewer 3 Report
In this paper, the authors report the synthesis and characterization of black TiO2 film using hollow cathode hydrogen plasma treatment (HCHP). They also studied the photocatalytic activity of black TiO2 by the degradation of dye Methylene blue. The introduction and aims are clear. Overall the results are clear and well written.
I have the following minor suggestions for the paper.
The significance and advantage of using the HCHP for the synthesis should be discussed in the introduction. The authors should compare the photocatalytic property and the growth parameters of HCHP reported by other authors. They should also discuss the advantages of their parameters. Whether they reducted the growth temperature, less time, etc. Slightly elaborate the figure caption so that they can be understood independently of the text, if possible. Figure 7 B, the scale bar is missing.
Author Response
We would like to thank the reviewers for their comments / suggestions and we hope that this revised version of our manuscript – that takes into consideration the reviewers’ suggestions – meets the requirements for publication in the Catalysts Journal. Changes were made using Microsoft Word "track changes" feature, as recommended, and texts added to the manuscript are highlighted in yellow.
Best regards,
The authors
Response to the Reviewer's comments and list of changes made to the manuscript:
Reviewer 3
1. “The significance and advantage of using the HCHP for the synthesis should be discussed in the introduction.”
Response: We thank the reviewer for this observation and we agree that this information is important to be highlighted. Therefore, we inserted in the “Introduction” section this paragraph (lines 108-123):
“As an alternative to expensive high-density plasma sources such as ICP and electron cyclotron resonance (ECR) plasma, hollow cathode-based plasmas have arisen [30,31]. These plasmas are attractive due to a simple modification of the biased electrode (cathode), from the usual plane geometry to a hollow configuration which significantly enhances the ionization efficiency of the gas discharge at lower pressures. This occurs because of electrostatic trapping of electrons in the discharge volume called hollow cathode effect (HCE), which promotes a drastic increase in ionization and dissociation processes [30-33]. Various applications such as thin film deposition [30], material etching [31], activation of surfaces [30], have benefited from this plasma technology. Thus, the hollow cathode hydrogen plasma (HCHP) can be interesting for blackening TiO2 material application due to its potential and lack of studies in the literature. Indeed, as it enhances the generation of hydrogen radicals and ions and, for samples placed on biased electrode, increases the efficiency of hydrogen incorporation in the lattice material, as well as to modifying the surface morphology due to the incidence of high energy hydrogen ions. All phenomenology of HCHP allows for short treatment times and because of the good plasma uniformity, allows not only the blackening of thin films but also nano powders.”
2. “The authors should compare the photocatalytic property and the growth parameters of HCHP reported by other authors. They should also discuss the advantages of their parameters. Whether they reducted the growth temperature, less time, etc”
Response: We thank the reviewer for this suggestion. Even the results of the photocatalysis experiments are particular to each work due to the differences between the light sources, contaminant dye and layout reactors used in each one, we added a comparison between our black TiO2 and the one produced in the reference 18, since this reference presented the fastest method to produce this material and used the same dye in the photocatalysis experiments (methylene blue). The comparative was made in terms of the kinetics of the degradation reaction k, as can be seen in section 2.4 from line 341 to 360 (highlighted in yellow), emphasizing the advantages of our process.
3. “Slightly elaborate the figure caption so that they can be understood independently of the text, if possible.”
Response: We thank the reviewer for this recommendation. We have modified the figure captions as much as possible to meet this recommendation. The changes are highlighted in yellow.
4. “Figure 7 B, the scale bar is missing.”
Response: We thank the reviewer for observing this issue. Figure 7b was corrected.
Reviewer 4 Report
Please, publish this work subject to the following revisions:
The authors say that black titania possesses superior photocatalytic properties. Their results support this statement. However, not all black titanias are superior photocatalysts. Perhaphs the authors should elaborate on this issue and make clear what is the origin of the present superiority. The authors also say that their product mainly absorbs in the UV. Is the color change affecting photocatalytic properties of titania or does color originate from a few sites which are photocatalytically inert?
I do not undestand the purpose of Figure 10 and please explain what is photocatalyst light absorbance profile during photocatalysis. In other words, which spectral response affects photocatalysis, UV, vis, or IR? Is it possible that enhanced photocatalysis may derive from photocatalyst structure modification as indicated by Figure 1?
Author Response
We would like to thank the reviewers for their comments / suggestions and we hope that this revised version of our manuscript – that takes into consideration the reviewers’ suggestions – meets the requirements for publication in the Catalysts Journal. Changes were made using Microsoft Word "track changes" feature, as recommended, and texts added to the manuscript are highlighted in yellow.
Best regards,
The authors
Response to the Reviewer's comments and list of changes made to the manuscript:
Reviewer 4
1. “The authors say that black titania possesses superior photocatalytic properties. Their results support this statement. However, not all black titanias are superior photocatalysts. Perhaps the authors should elaborate on this issue and make clear what is the origin of the present superiority.”
Response: We thank the reviewer for this observation and take the opportunity to clarify this point further. Therefore, we added and improved the text in section 2.4 (more precisely from line 339 to 358) to highlight the reasons for photocatalytic improvement.
2. “The authors also say that their product mainly absorbs in the UV. Is the color change affecting photocatalytic properties of titania or does color originate from a few sites which are photocatalytically inert?”
Response: The color change is only an indicative that our film absorbs in the visible range after HCHP treatment. UV absorption is an intrinsic characteristic of anatase TiO2 materials. For this reason, our pristine TiO2 thin film presented a great absorption performance in this range, as well as our black TiO2 film. As the light source used for the photocatalytic experiments emits light in the UV-Vis range and the pristine TiO2 absorbs light only in the UV range, the increased absorption in the visible range promoted by the HCHP treatment contributed to the better photocatalytic performance of the black TiO2. This is now better discussed in section 2.4 (highlighted in yellow from line 339 to 358) after the suggestions / recommendations proposed by the reviewers.
3. “I do not understand the purpose of Figure 10 and please explain what is photocatalyst light absorbance profile during photocatalysis. In other words, which spectral response affects photocatalysis, UV, vis, or IR? Is it possible that enhanced photocatalysis may derive from photocatalyst structure modification as indicated by Figure 1?”
Response: Figure 10 is important to support the discussion of the section related to the results of photocatalysis (section 2.4). In bring this figure closer to the results and discussions about the photocatalysis process, we changed its location in the article and it is now in Figure 9c. It is necessary to clarify at what wavelengths the light source used in the photocatalysis experiments emits to precisely state that the improvement in photocatalytic activity is due to the increase in absorption in a given range of the electromagnetic spectrum. In our case, this improvement was due to the increase in absorption in the visible range, since the pristine sample absorbs only in the UV range and, in addition, the black sample absorbs the same amount in the UV, besides absorbing in the VIS range (see Fig 7a). We cannot affirm that the increase in absorption in the NIR range contributed to the better performance of the photocatalytic process due to our light source (as shown in Fig. 9c) does not emit radiation in the NIR range. In addition, the increases in surface area of ~24% indicated by the AFM results (Figure 1 and Table 1) also contributed to the better degradation efficiency of methylene blue, since with a larger area in contact with the dye, greater is the amount of dye degraded by the active sites of the black TiO2 film surface. All this information is now better discussed in section 2.4 (highlighted in yellow from line 339 to 358).
Reviewer 5 Report
General Comments:
Black TiO2 films are prepared using the hollow cathode hydrogen plasma (HCHP) technique from pristine anatase TiO2 films created on silicon and cover glass substrates. The work includes characterization analyses by AFM, XRD, UV-Vis spectrophotometry, Raman spectroscopy, and XPS techniques. Photocatalytic measurements were explored using a methylene blue (MB) UV degradation method that is not reliable (as the work of the references cited in the manuscript) for the reasons explained below in more detail. The black TiO2 is highly absorbing in the UV-visible region. More experimental work is needed to support the conclusions of performance as a photocatalytic agent. Therefore, a major revision is recommended providing the authors a minimum of 30 days to comprehend what needs to be done and perform it correctly.
Major Comments:
l. 35: Why is it stated in the abstract that the black TiO2 has potential for hydrogenation only in “short periods”? Is it because the material is unstable and no reproducibility tests has been reported? If the material gets decomposed would not be a catalyst.
l. 40-43. Following this long general statement lacking references, it would be convenient to include in the manuscript the following photocatalysis references that have integrated the concept of remediating pollution with improving the environment in a novel way:
Ikeda et al., ACS APPLIED ENERGY MATERIALS (2019), Volume: 2 Issue 9, Pages: 6911-6918. DOI: 10.1021/acsaem.9b01418.
Zhou et al., JOURNAL OF PHYSICAL CHEMISTRY C (2014), Volume: 118 Issue: 22 Pages: 11649-11656. DOI: 10.1021/jp4126039.
Nogueira et al., SCIENTIFIC REPORTS (2019), Volume: 9 Issue 1, DOI: 10.1038/s41598-018-36683-8.
l. 57: Before starting line 58, it would be really important to highlight that heterojunctions by coupling two semiconductors are a great method to improve the photocatalytic activities. For this point, the following two key articles that demonstrated how to build and distinguish the kind of heterojunctions should be included in the manuscript:
Li et al., CERAMICS INTERNATIONAL (2017), Volume 43, Pages: 16007-16012, doi: 10.1016/j.ceramint.2017.08.021.
Aguirre et al., APPLIED CATALYSIS B: ENVIRONMENTAL (2017), Volume 217, Pages: 485-493, doi: 10.1016/j.apcatb.2017.05.058.
l. 77-95: Have the references cites (and this manuscript) consider the importance of changing light absorption? Performing the actinometry is key to address this point. Otherwise, all observations could just be due to an absorption enhancement that could be accounted appropriately in a comparison of the quantum efficiencies of pure and black TiO2.
l. 115-116, l. 288-318 (section 2.4, Figure 8), l. 381-395 (section 3.4): The process of photocatalysis is more complicated that assumed in this manuscript. When attempting to describe the photocatalytic activity, the manuscript is missing the following recent references providing the expertise of what to report, which both needs to incorporated into the text:
Hoque, MATERIALS (2018), Volume 11 Issue 10, 1990; doi: 10.3390/ma11101990.
Buriak, CHEM. MATER. (2015), Volume 27 Issue 4911; doi: 10.1021/acs.chemmater.5b02323.
In more detail, the experimental work needs some improvements conceptually and more measurements for the completion of the tests. The manuscript is missing key information that needs to be provided. For example, what are the apparent quantum efficiencies (AQE) for pristine and black TiO2? The actinometry work is missing and will be needed to provide the AQE required to account for the number of photons that drive the photocatalytic reaction. Otherwise, the information provided is useless. Furthermore, the references indicate that methylene blue decomposition is not a good test for photocatalytic activity but that, e.g., phenol decomposition has been recommended as a test.
l. 119: TiO2 is not transparent.
l. 140: What is the scanning rate of the instrument (not provided in the experimental section)? What is the error of the experimental measurement in 0.1 degrees?
l. 144, Table 1 and Figure 2: The text appears to contradict what the figure shows for peak 101 as the full with at half the maximum appear to be identical. Are the changes in lattice parameters in Table 1 within the error of the method? It is weird that the XRD in Figure 2 look practically the same for pristine and black TiO2 considering the large changes in morphology registered by AFM in Figure 1.
l. 198: Unclear text: “molecules volatile”.
l. 211: Explain how hydroxyl radical is loss as a volatile or reconsider the reaction.
l. 218: Substitute “experiments” by “measurements”.
l. 221-231 (Figure 6 and associated text): Pristine TiO2 should be transparent when it is shown in panel a to absorb ~20 of light (> 370 nm), suggesting something is wrong with the measurements. There is a lack of values that could show where “0” is in panels b and c. This is important to observe where the bandgap is and report it to the second decimal. Both materials have different bandgaps in contradiction to what is written and shown in the text. After editing the text accordingly, Figure 7 may need to be re-evaluated too.
l. 286: It may be missing the unit cm instead of the square symbol.
l. 396-409: Revise the conclusions once all issues above have been solved.
Author Response
We would like to thank the reviewers for their comments / suggestions and we hope that this revised version of our manuscript – that takes into consideration the reviewers’ suggestions – meets the requirements for publication in the Catalysts Journal. Changes were made using Microsoft Word "track changes" feature, as recommended, and texts added to the manuscript are highlighted in yellow.
Best regards,
The authors
Response to the Reviewer's comments and list of changes made to the manuscript:
Reviewer 5
1. “35: Why is it stated in the abstract that the black TiO2 has potential for hydrogenation only in “short periods”? Is it because the material is unstable and no reproducibility tests has been reported? If the material gets decomposed would not be a catalyst.”
Response: We believe that is a misunderstood of the final part of the abstract: “These properties of black TiO2 film, as well as its performance as a photocatalytic agent, were investigated, indicating the superior quality of this material in thin film form and the promising potential of the HCHP treatment for TiO2 hydrogenation in short periods.”, which is intended to say that “HCHP treatment has potential to produce hydrogenated TiO2 in very short process time”. We apologize to the reviewer for this misunderstanding. One of the objectives of this work is to emphasize that we were able to obtain black TiO2 quickly (15 min) and efficiently in relation to the other techniques that are reported in the literature. In order to avoid further confusion, the referred text is now rewritten accordingly (highlighted in yellow).
2. “40-43. Following this long general statement lacking references, it would be convenient to include in the manuscript the following photocatalysis references that have integrated the concept of remediating pollution with improving the environment in a novel way:
Ikeda et al., ACS APPLIED ENERGY MATERIALS (2019), Volume: 2, Issue 9, Pages: 6911-6918. DOI: 10.1021/acsaem.9b01418.
Zhou et al., JOURNAL OF PHYSICAL CHEMISTRY C (2014), Volume: 118 Issue: 22 Pages: 11649-11656. DOI: 10.1021/jp4126039.
Nogueira et al., SCIENTIFIC REPORTS (2019), Volume: 9 Issue 1, DOI: 10.1038/s41598-018-36683-8.”
Response: We thank the reviewer for the suggestion. We added these references to the text (see line 43, highlighted in yellow).
3.“57: Before starting line 58, it would be really important to highlight that heterojunctions by coupling two semiconductors are a great method to improve the photocatalytic activities. For this point, the following two key articles that demonstrated how to build and distinguish the kind of heterojunctions should be included in the manuscript:
Li et al., CERAMICS INTERNATIONAL (2017), Volume 43, Pages: 16007-16012, doi: 10.1016/j.ceramint.2017.08.021.
Aguirre et al., APPLIED CATALYSIS B: ENVIRONMENTAL (2017), Volume 217, Pages: 485-493, doi: 10.1016/j.apcatb.2017.05.058.”
Response: We agree and thank the reviewer for the suggestion. A comment regarding semiconductor heterojunctions and the suggested biography was added to the manuscript (from line 57 to 59, highlighted in yellow).
4. “77-95: Have the references cites (and this manuscript) consider the importance of changing light absorption? Performing the actinometry is key to address this point. Otherwise, all observations could just be due to an absorption enhancement that could be accounted appropriately in a comparison of the quantum efficiencies of pure and black TiO2.”
Response: We agree and thank the reviewer for elucidating this very important point. However, this is not the focus of this work. All the information about the optical properties of the films presented in this study were obtained by transmittance and reflectance measurements. Unfortunately, we do not have the equipment necessary to perform actinometry measures. However, we will look for collaboration with other research groups in order to perform actinometry measurements for future works.
5. “l. 115-116, l. 288-318 (section 2.4, Figure 8), l. 381-395 (section 3.4): The process of photocatalysis is more complicated that assumed in this manuscript. When attempting to describe the photocatalytic activity, the manuscript is missing the following recent references providing the expertise of what to report, which both needs to incorporated into the text:
Hoque, MATERIALS (2018), Volume 11 Issue 10, 1990; doi: 10.3390/ma11101990.
Buriak, CHEM. MATER. (2015), Volume 27 Issue 4911; doi: 10.1021/acs.chemmater.5b02323.”
Response: We agree with the referee and appreciate the comment and the references. The idea of this work is to present to the scientific community the new production process of Black-TiO2 and its main properties, as well as to raise hypotheses of how the formation of this new material occurs. The photocatalytic test presented is not the focus of the work and was presented only to show an increase in the degradation of the dye when exposed to germicidal lamps in the presence of Black-TiO2. To ensure the comparison, the tests performed on the “pristine TiO2” and the “black TiO2” were performed under the same conditions. This information was made clearer in the manuscript. In addition, the next step of this work is to conduct new and more systematic photocatalytic tests based on what is presented in the Hoque (2018) and Buriak (2015) works. We added this pretension regarding future works along with the references suggested at the end of section 3.4 (see line 432, highlighted in yellow).
6. “In more detail, the experimental work needs some improvements conceptually and more measurements for the completion of the tests. The manuscript is missing key information that needs to be provided. For example, what are the apparent quantum efficiencies (AQE) for pristine and black TiO2? The actinometry work is missing and will be needed to provide the AQE required to account for the number of photons that drive the photocatalytic reaction. Otherwise, the information provided is useless. Furthermore, the references indicate that methylene blue decomposition is not a good test for photocatalytic activity but that, e.g., phenol decomposition has been recommended as a test.”
Response: We thank again the reviewer; these are a very important point to address in a future work dedicated to photocatalysis. As mentioned before, the focus of this work is to address the process of TiO2 blackening and its main resulting characteristics, based on a very rich set of characterization techniques.
7. “119: TiO2 is not transparent.”
Response: This is just semantics. “Transparent” in this work is a short term to refer to “Hight transmittance (above 95% in relation to the substrate) under visible light spectrum”. See more discussion regarding this issue in point 13. Even so, we removed the word “transparent” form the text in order to avoid misunderstanding in this regard.
8. “140: What is the scanning rate of the instrument (not provided in the experimental section)? What is the error of the experimental measurement in 0.1 degrees?”
Response: To perform the XRD measurements were used a step of 0.013o and a measurement time of 60 s/step. This information was added to section 3.3 (see line 404, highlighted in yellow). We agree with the reviewer that the 0.1 degrees variation observed in the main peak of TiO2 after hydrogenation may be within the experimental error of the measurement. In order to prove this variation, we should make more detailed diffraction measurements, like neutron diffraction. For this reason, we have rewritten the paragraph where we discussed the XRD measures and remove the lattice parameters of Table 1.
9. “144, Table 1 and Figure 2: The text appears to contradict what the figure shows for peak 101 as the full with at half the maximum appear to be identical. Are the changes in lattice parameters in Table 1 within the error of the method? It is weird that the XRD in Figure 2 look practically the same for pristine and black TiO2 considering the large changes in morphology registered by AFM in Figure 1.”
Response: As discussed in the text, the change in the morphology (AFM) is mainly due to the corrosion of surface grain boundaries, which is mostly amorphous. Thus, no major modifications are expected in the XRD patterns, once no major structural modifications are taking place into the high crystallized fraction of the bulk crystallites. Even so, in order to improve understanding as discussed in the last question, the text in this part were rewritten and we remove the lattice parameters of Table 1.
10. “198: Unclear text: “molecules volatile”.”
Response: We thank the reviewer to notice that. The correct sentence is “volatile molecules”, which was corrected in the text (see line 228, highlighted in yellow).
11. “211: Explain how hydroxyl radical is loss as a volatile or reconsider the reaction.”
Response: The hydroxyl groups, as well as the H2O volatile molecules, were possibly formed from the interaction of the H+ ions (coming from the hydrogen plasma) with the surface of the film, as was presented in the manuscript from the line 227 to 229 with the aid of equations 1-3 and Fig. 2a. We understand that a more detailed plasma study must be performed in future works to try to explain better the interactions between the H2 plasma and the TiO2 film surface. Hence, this consideration was added to the text in the final of the paragraph where this issue is presented (see line 231, highlighted in yellow).
12. “218: Substitute “experiments” by “measurements”.”
Response: We thank the reviewer for this suggestion. We made the word change, as can be seen in section 2.3 line 245, highlighted in yellow.
13. “221-231 (Figure 6 and associated text): Pristine TiO2 should be transparent when it is shown in panel a to absorb ~20 of light (> 370 nm), suggesting something is wrong with the measurements. There is a lack of values that could show where “0” is in panels b and c. This is important to observe where the bandgap is and report it to the second decimal. Both materials have different bandgaps in contradiction to what is written and shown in the text. After editing the text accordingly, Figure 7 may need to be re-evaluated too.”
Response: Pristine TiO2 is transparent in the visible light spectrum. The maximums of the transmittance interference fringe (caused by the multiple reflections in the interior of the film) reach the same values as the pure substrate, which is also transparent, with ~90% of transmittance in this spectrum. The missing 10% is due to the reflection (specular + diffuse). With. In order to clarify this point, the substrate T% spectrum was included in Figure 6 (now figure 7).
14. "286: It may be missing the unit cm instead of the square symbol."
Response: It is now corrected. We thank the reviewer again.
15. “396-409: Revise the conclusions once all issues above have been solved.”
Response: The conclusion was properly revised.
Round 2
Reviewer 4 Report
Please, publish this revised version
Author Response
We would like to thank the reviewer for comments / suggestions.
Best regards
The authors
Reviewer 5 Report
General Comments:
Major improvements are observed in the revisions as the result of partially addressing the original comments. However, there are issues remaining that are problematic in this new manuscript, as the additional experimental work recommended has not been completed to support the conclusions of performance for black TiO2 as a photocatalytic agent. Therefore, a major revision is recommended again.
Major Comments:
The work has not considered how to account for the number of photon that drive the reaction for each pristine and black TiO2. Performing an actinometry does not require additional equipment as the authors already have a UV-vis spectrometer. More can be read in the references previously provided and in the literature there cited. The observations reported could just be due to an absorption enhancement that could be accounted appropriately in a comparison of the apparent quantum efficiencies of pure and black TiO2. The process of photocatalysis is more complicated that assumed in this manuscript as indicated before. In more detail, the experimental work needs some improvements conceptually and more measurements for the completion of the tests. The manuscript is missing key information that needs to be provided. What are the apparent quantum efficiencies (AQE) for pristine and black TiO2? The actinometry work is missing and will be needed to provide the AQE required to account for the number of photons that drive the photocatalytic reaction. Otherwise, the information provided is useless. Furthermore, the references indicate that methylene blue decomposition is not a valid test for photocatalytic activity but that, e.g., phenol decomposition has been recommended as a test. The characterizations reported are of no use here, as the key for a paper in catalysis, about a photocatalyst, is to demonstrate its photocatalytic activity.
227 (and l. 241): OH is not a molecule. The statement is still unclear. The technique cannot observe radicals. The explanation of how hydroxyl radical is loss as a volatile is still confusing as is the chronological connection of events described.
Figure 7 and associated text: Pristine TiO2 should be transparent when it is shown in panel a to absorb ~20 of light (> 370 nm), suggesting something is wrong with the measurements. There is a lack of values that could show where “0” is in panels b and c. The explanation provided:
“Pristine TiO2 is transparent in the visible light spectrum. The maximums of the transmittance interference fringe (caused by the multiple reflections in the interior of the film) reach the same values as the pure substrate, which is also transparent, with ~90% of transmittance in this spectrum. The missing 10% is due to the reflection (specular + diffuse). With. In order to clarify this point, the substrate T% spectrum was included in Figure 6 (now figure 7).” needs to be added to the text.
There was no answer to the previous comment to Figure 7 panels b and c: There is a lack of values that could show where “0” is in the y-axes of panels b and c. This is important to observe where the bandgap is and report it to the second decimal. Both materials have different bandgaps in contradiction to what is written and shown in the text. After editing the text accordingly, Figure 8 may need to be re-evaluated too.”
This is important to observe where the bandgap is and report it to the second decimal. Both materials have different bandgaps in contradiction to what is written and shown in the text. After editing the text accordingly, Figure 7 may need to be re-evaluated too.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Round 3
Reviewer 5 Report
The authors have done a tremendous effort to improve the manuscript during the revision. Please keep in mind that the original revision stated that authors would need a minimum of 30 days to complete the work. Even if given less time, the authors are welcome to request a deadline extension to learn what they need to do and execute it. There are still some remaining problems with the use of methylene blue (and the justification in the response is not correct as the references cited are also wrong). The paper should have included the work with Phenol, which the authors claim to proceed faster for black TiO2 than for pristine. The use a single wavelength to estimate AQE with polychromatic irradiation is also incorrect. The y-axes is still not showing the "0" value for each Tauc plot as requested 2-times already. Despite these major deficiencies, reviewer consider the manuscript could be publish now.
