P-type (CuTi)Ox Thin Films Deposited by Magnetron Co-Sputtering and Their Electronic and Hydrogen Sensing Properties

Round 1

Reviewer 1 Report

This article was submitted by Ewa Makowska, Micha Mazur, Jaroslaw Domaradzki, and Damian Wojcieszak under the title: "P-type (CuTi)Ox thin films deposited by magnetron co sputtering and their electronic and hydrogen sensing properties.". 

These authors investigated a number of properties for copper and titanium oxide mixtures. In order to achieve the goal of the paper, magnetron co-sputtering was used to prepare (CuTi)Ox thin films with varying Cu concentrations. 

Please find my comments here.

1. If you are going to list the parameters for the deposition, it would be helpful to include the unit of power. 

2. In regard to the rate of film deposition, there has been no information provided to the reader. Could you please provide the deposition rate of the films and how these values were measured? 

3. It may be possible for the authors to prove the chemical composition of their alloys by using X-ray photoelectron spectroscopy. If not, please do not consider it. 

4. It would be helpful if you could provide more information about hydrogen gas sensing mechanism used by the samples. 

Author Response

Answers to the report of Reviewer #1

on the manuscript entitled: “P-type (CuTi)Ox thin films deposited by magnetron co-sputtering and their electronic and hydrogen sensing properties”

Authors: Ewa Mańkowska, Michał Mazur, Jarosław Domaradzki, Damian Wojcieszak

Authors:

We would like to express our gratitude for your remarks, which let us improve our manuscript. We have taken them into account in the revised version of our paper.

Answering to the Reviewer’s remarks, we have introduced some revisions in the manuscript.

Reviewer:

If you are going to list the parameters for the deposition, it would be helpful to include the unit of power. 

Authors:

Authors thank for the remark, the unit of power applied to target is watt and the table 1 was completed with the missing unit as follows:

Table 1. Summary of the main process parameters for magnetron co-sputtering deposition of copper oxide (Cu_xO), titanium oxide (TiO_x), and mixed copper and titanium oxides (CuTi)Ox.

Reviewer:

In regard to the rate of film deposition, there has been no information provided to the reader. Could you please provide the deposition rate of the films and how these values were measured? 

Authors:

The deposition rate was added to Table 1 and was calculated based on the deposition time and the thin film thickness measured using optical profilometer.

Reviewer:

• It may be possible for the authors to prove the chemical composition of their alloys by using X-ray photoelectron spectroscopy. If not, please do not consider it.
• It would be helpful if you could provide more information about hydrogen gas sensing mechanism used by the samples.

Authors:

The authors have already performed x-ray photoelectron spectroscopy studies with regard to as-deposited and annealed thin films. These results are going to be analysed in future articles, as it is necessary for the explanation of the gas sensing mechanisms. Thorough analysis of the influence of hydrogen in various concentrations on the electrical properties is currently under development, i.e. measurements of H₂ concentrations of 200, 500 and 1000 ppm were performed. Moreover, XPS studies were done not only for as-deposited and annealed thin films, but also for some specific experiments. The authors measured the chemical state of the surface in-situ using XPS before and after exposition of the sample to hydrogen. These measurements allowed for the determination of the change in oxidation state of the investigated thin films. Measurements were made before and after exposure to H₂. Measurements made after exposition were made for exposure times of 15 minutes and an additional 45 minutes. Those results clearly showed that the surface of the thin films undergoes the reduction process from CuO or mixed CuO-Cu₂O to pure Cu₂O, which is also a part of the mechanism responsible for changes in changes in electrical resistance shown in the proper graphs. The authors are showing some of the results below (taken directly from the CasaXPS software). It is worth mentioning that those results will be shown in upcoming articles that will be mainly focused more on the hydrogen gas sensing properties.

Results of the XPS measurements of Cu_xO thin films annealed at 200 °C for Cu2p region:
a) before and b) after exposure to hydrogen

Reviewer 2 Report

Manuscript "coatings-2119819" needs minor revision considering following issues:

1-Interpretation of XRD pattern results needs to be improved. For example the sentence “Thin films containing 23 at. %, 41at. %, and 56 at. % of copper were monocrystalline, contrary to (Cu0.77Ti0.23)Ox, which was polycrystalline. “is not correct! since grains(particles) can be observed in SEM image of all samples moreover XRD pattern of thin film recorded with normal configuration can net determine if a film is single crystalline or not.

2-Based on the XRD results, some information about texture or preferred orientation of films can also be obtained.

3-Tables need to be reformatted

4-Caption of Figure 2 needs to be amended. I suggest describing that P denotes the applied power of sputtering.

5-In my survey, other group also prepared Cu/TiO2 films by sputtering and in some cases they perform post annealing of the film. Therefore, I suggest to revisit introduction considering the published literature (e.g. following articles)

Zhao, Zhiming, et al. "The study of microstructure, optical and photocatalytic properties of nanoparticles (NPs)-Cu/TiO2 films deposited by magnetron sputtering." Journal of Alloys and Compounds 652 (2015): 307-312.

Sreedhar, M., et al. "Cu/TiO2 thin films prepared by reactive RF magnetron sputtering." Applied Physics A 120.2 (2015): 765-773.

Mekasuwandumrong, Okorn, et al. "Synthesis of Cu/TiO2 catalysts by reactive magnetron sputtering deposition and its application for photocatalytic reduction of CO2 and H2O to CH4." Ceramics International 45.17 (2019): 22961-22971.

Fouad, S. S., et al. "Microstructural and optical duality of TiO2/Cu/TiO2 trilayer films grown by atomic layer deposition and DC magnetron sputtering." Inorganic Chemistry Communications 145 (2022): 110017.

6-data of gas sensing can be correlated with phase composition of samples.

7-Text needs proofreading to remove minor language errors. For example, check highlighted text in attached version of manuscript.

Author Response

Answers to the report of Reviewer #2

on the manuscript entitled: “P-type (CuTi)Ox thin films deposited by magnetron co-sputtering and their electronic and hydrogen sensing properties”

Authors: Ewa Mańkowska, Michał Mazur, Jarosław Domaradzki, Damian Wojcieszak

Authors:

We would like to express our gratitude for your remarks, which let us improve our manuscript. We have taken them into account in the revised version of our paper.

Answering to the Reviewer’s remarks, we have introduced some revisions in the manuscript.

Reviewer:

Interpretation of XRD pattern results needs to be improved. For example the sentence “Thin films containing 23 at. %, 41at. %, and 56 at. % of copper were monocrystalline, contrary to (Cu0.77Ti0.23)Ox, which was polycrystalline. “is not correct! since grains(particles) can be observed in SEM image of all samples moreover XRD pattern of thin film recorded with normal configuration can net determine if a film is single crystalline or not. 

Authors:

This sentence was removed from the manuscript.

Reviewer:

Based on the XRD results, some information about texture or preferred orientation of films can also be obtained 

Authors:

Authors calculated Lotgering factor [1] to verify if any texturization or preferred orientation occurs in investigated thin films, however no preferred orientation was found as the Lotgering factor was in the range of 0.1 to 0.3. For (Cu_0.77Ti_0.23)Ox  and Cu_xO thin films annealed at 200°C the Lotgering factor calculated for (111) was equal to 0.12 what testifies to poor orientation [2].

1. Brosnan, K.H.; Messing, G.L.; Meyer, R.J.; Vaudin, M.D. Texture Measurements in 〈001〉Fiber-Oriented PMN-PT. J. Am. Ceram. Soc. 2006, 89, 1965–1971, doi:10.1111/j.1551-2916.2006.01049.x.
2. Lotgering, F.K. Topotactical Reactions with Ferrimagnetic Oxides Having Hexagonal Crystal Structures-I. J. Inorg. Nucl. Chem. 1959, 9, 113–123, doi:10.1016/0022-1902(59)80070-1.

Reviewer:

Tables need to be reformatted.

Authors:

Tables were reformatted to the Coatings journal style.

Reviewer:

Caption of Figure 2 needs to be amended. I suggest describing that P denotes the applied power of sputtering.

Authors:

Caption of Figure 2 has been corrected on:

Figure 2. Dependence of copper content in as-deposited thin films on the power ratio P_Cu/(P_Cu+P_Ti) applied to magnetrons with Cu and Ti targets.

Reviewer:

In my survey, other group also prepared Cu/TiO2 films by sputtering and in some cases they perform post annealing of the film. Therefore, I suggest to revisit introduction considering the published literature (e.g. following articles):

• Zhao, Zhiming, et al. "The study of microstructure, optical and photocatalytic properties of nanoparticles (NPs)-Cu/TiO2 films deposited by magnetron sputtering." Journal of Alloys and Compounds652 (2015): 307-312.
• Sreedhar, M., et al. "Cu/TiO2 thin films prepared by reactive RF magnetron sputtering." Applied Physics A2 (2015): 765-773.
• Mekasuwandumrong, Okorn, et al. "Synthesis of Cu/TiO2 catalysts by reactive magnetron sputtering deposition and its application for photocatalytic reduction of CO2 and H2O to CH4." Ceramics International17 (2019): 22961-22971.
• Fouad, S. S., et al. "Microstructural and optical duality of TiO2/Cu/TiO2 trilayer films grown by atomic layer deposition and DC magnetron sputtering." Inorganic Chemistry Communications145 (2022): 110017.

Authors:

Authors are grateful for the suggesting some articles on the topic of the modification of TiO₂ with copper. The introduction has been extended to include the indicated articles.

“Incorporation of Cu_xO into the TiO₂ matrix improved photocatalytic activity in the case of pollution decomposition [24–27] including CO₂ reduction [28] and hydrogen generation in the photoinduced water splitting process [15,29–32]. Antimicrobacterial studies show that copper addition to TiO₂ has a positive effect in reducing bacterial (e.g. E. coli) viability [2,33].”

Reviewer:

Data of gas sensing can be correlated with phase composition of samples.

Authors:

These results are going to be analysed in future articles, as it is necessary for the explanation of the gas sensing mechanisms. The authors have already performed gas sensing measurements of H₂ concentrations of 200, 500 and 1000 ppm and the chemical state of the surface have been measured using XPS. Moreover, in-situ XPS measurements, before and after exposition of the sample to hydrogen were performed to determine the change in oxidation state of the investigated thin films. Thorough analysis of the influence of hydrogen in various concentrations on the electrical properties of (CuTi)Ox thin films with various copper concentration is currently under development. Those results will be shown in upcoming articles that will be mainly focused more on the hydrogen gas sensing properties.

Reviewer:

Text needs proofreading to remove minor language errors. For example, check highlighted text in attached version of manuscript.

Authors:

Authors are thankful for the comment. Article was corrected to remove language errors.

Author Response File: Author Response.pdf

Reviewer 3 Report

The author deposited thin films of copper oxide (CuxO), titanium oxide (TiOx), and several mixtures of copper 8 and titanium oxides ((CuTi)Ox) using magnetron sputtering for gas sensing. The authors have done lots of work. The work can be published after addressing the following questions.

1.     From the comparison of this work with other work in literature, what’s the advantage of the work?

2.     Did the author try to find the minimum and maximum concentration of hydrogen that is able to test with the device. It’s necessary to tell the audience of the best performance.

3.     The authors have fabricated materials with various ratios which provide different performance. What’s the mechanism behind it. Please explain.

4.     What’s the influence of the different morphology shown in Figure 3?

Author Response

Answers to the report of Reviewer #3

on the manuscript entitled: “P-type (CuTi)Ox thin films deposited by magnetron co-sputtering and their electronic and hydrogen sensing properties”

Authors: Ewa Mańkowska, Michał Mazur, Jarosław Domaradzki, Damian Wojcieszak

Authors:

We would like to express our gratitude for your remarks, which let us improve our manuscript. We have taken them into account in the revised version of our paper.

Answering to the Reviewer’s remarks, we have introduced some revisions in the manuscript.

Reviewer:

From the comparison of this work with other work in literature, what’s the advantage of the work? 

Authors:

According to literature review there is no work on mixed oxides of copper and titanium thin films where copper/titanium concentration was exanimated in such wide rage. In other works, copper concentration does not exceed 20%. Moreover, in most cases copper is only metallic. The authors believe that fabrication this type of material and determining its properties is important, because even the Cu-Ti-O phase diagram is not well characterised. One of the greatest strengths of the work is characterisation of how temperature the oxidation of copper and titanium process proceeds depending on annealing. Moreover, the work covers a wide rage of properties (optical, electrical and gas sensing) that have rarely been described in compact way before. It was found out that mixed copper and titanium oxides thin films might exhibit enhanced response toward hydrogen comparing to both separate  oxides. In this work, mixed oxide with 77 at. % of copper showed one of the highest reported response to hydrogen. Additionally, authors have performed  hydrogen  sensing measurements of hydrogen concentrations lower that 1000 ppm but still gas sensing mechanism of (CuTi)Ox thin films is still under development.

Reviewer:

• Did the author try to find the minimum and maximum concentration of hydrogen that is able to test with the device. It’s necessary to tell the audience of the best performance. 
• The authors have fabricated materials with various ratios which provide different performance. What’s the mechanism behind it. Please explain.

Authors:

The authors have already performed x-ray photoelectron spectroscopy studies with regard to as-deposited and annealed thin films. These results are going to be analysed in future articles, as it is necessary for the explanation of the gas sensing mechanisms. Thorough analysis of the influence of hydrogen in various concentrations on the electrical properties is currently under development, i.e. measurements of H₂ concentrations of 200, 500 and 1000 ppm were performed. Moreover, XPS studies were done not only for as-deposited and annealed thin films, but also for some specific experiments. The authors measured the chemical state of the surface in-situ using XPS before and after exposition of the sample to hydrogen. These measurements allowed for the determination of the change in oxidation state of the investigated thin films. Measurements were made before and after exposure to H₂. Measurements made after exposition were made for exposure times of 15 minutes and an additional 45 minutes. Those results clearly showed that the surface of the thin films undergoes the reduction process from CuO or mixed CuO-Cu₂O to pure Cu₂O, which is also a part of the mechanism responsible for changes in changes in electrical resistance shown in the proper graphs. The authors are showing some of the results below (taken directly from the CasaXPS software). It is worth mentioning that those results will be shown in upcoming articles that will be mainly focused more on the hydrogen gas sensing properties.

Results of the XPS measurements of Cu_xO thin films annealed at 200 °C for Cu2p region:
a) before and b) after exposure to hydrogen

Reviewer:

What’s the influence of the different morphology shown in Figure 3?

Authors:

Morphology of the thin films strongly influences gas sensing properties, as porous materials offer easier gas diffusion, and more active adsorption sites on which gas sensing reactions occur. As it was explained it the article, typically TiO_x thin films exhibit better sensor response than Cu_xO. In this work, Cu_xO porous structure might explain the fact that Cu_xO thin film exhibited slightly higher SR value and moreover the morphology explained by and also explains longer recovery time comparing with TiO_x [X1].

“Metal oxides that exhibit the p-type of electrical conduction typically respond worse than their n-type counterparts [6,62], however, the slightly better Cu_xO response may be caused by a considerable difference in the morphology and structure of the two thin films.”

X1.     Chowdhury, N.K.; Bhowmik, B. Micro/Nanostructured Gas Sensors: The Physics behind the Nanostructure Growth, Sensing and Selectivity Mechanisms. Nanoscale Adv. 2021, 3, 73–93, doi:10.1039/d0na00552e.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The paper is now qualified for publish.