Photocatalytic Properties of Copper Nitride/Molybdenum Disulfide Composite Films Prepared by Magnetron Sputtering
Round 1
Reviewer 1 Report
Photocatalytic Properties of Copper Nitride/Molybdenum Disulfide Composite Films Prepared by Magnetron Sputtering
ID: Coatings-690747
The manuscript describes the preparation (by magnetron sputtering method) and characterization of Cu3N/MoS2 composite films. The films were tested as photocatalyst in the degradation of methyl orange in aqueous solution.
I have the following observations:
The authors must be consistent with the notations used, i.e. “Cu3N/MoS2 composite films” or “MoS2/Cu3N composite films”, “Cu target” or “Cu3N target”. The materials used in preparation of films and their producers must be provided. The authors discussed the variation of band gap energy. I suggest to calculate the values from experimental data and relate them to theoretical considerations. L65 “in a gas atmosphere of Ar gas and N2 at room temperature” – “gas” (in “Ar gas”) can be removed. L200 “When the power of MoS2 was 4 W” – it can be reformulated, for clarity (it is not “the power of MoS2”, but the powder at which it was obtained). “Combined with the results of XRD spectrum analysis, S and Mo were complexed with Cu3N in the form of a MoS2 compound” – Please reformulate. It is not a complexation process.
Author Response
Comment 1: The authors must be consistent with the notations used, i.e. “Cu3N/MoS2 composite films” or “MoS2/Cu3N composite films”, “Cu target” or “Cu3N target”.
Answer: Thank you very much for the reviewer’s tips, we have made changes in the text.
Comment 2: The materials used in preparation of films and their producers must be provided.
Answer: I'm sorry, because of our negligence, did not explain in the text. We make the following instructions: MoS2 target, Cu target(Beijing Jingmai Zhongke Material Technology Co., Ltd., Purity: 99.99%, Size: 60 ± 3 mm, Tolerance: +/- 0.1 mm)
Comment 3: The authors discussed the variation of band gap energy. I suggest to calculate the values from experimental data and relate them to theoretical considerations.
Answer: Thank you very much for your suggestions. We have added relevant data to the article based on your suggestions, as explained below:
Figure 1a shows the UV-Vis transmission spectra of Cu3N / MoS2 composite films prepared with different power values. The film showed good transmittance in the infrared region. With the increase of the MoS2 compounding amount, the transmittance of the film gradually increases. According to the transmission spectrum curve of the film and the law of optical constants (1), the absorption coefficient of the film can be calculated as follows:
(1) Among them, T represents transmittance, and d represents film thickness. Using the Tauc Equation2, the Eg of the film can be calculated:
(2) Where α, hv, and A represent the absorption coefficient, photon energy, and constant, respectively.
From the relationship curve of (αhv)2 and hv, calculate the band gap Eg by extrapolation, as shown in Figure 1b. The Eg range of Cu3N / MoS2 composite films is between 2.05-2.10 eV. As the power increases, the Eg of the composite film decreases first and then increases. When the deposition power is 4 W, the Eg of the film is the smallest, which is 2.05 eV. This may be because after MoS2 is recombined with Cu3N, Mo atoms replace Cu vacancies in the Cu3N lattice to act as donors, thus providing carriers to reduce the optical band gap; It is also possible that the film produced at a lower power has more defects. With the increase of power, the MoS2 content increases, and the optical band gap increases due to the hole filling effect in the valence band or the free carriers occupying the bottom of the conduction band.
Figure 1. UV-Vis transmission spectrum of Cu3N / MoS2 composite film
Comment 4： L65 “in a gas atmosphere of Ar gas and N2 at room temperature” –“gas” (in “Ar gas”) can be removed.
Answer: As reviewer suggested, we have modified the paragraph in our manuscript, as follows: “A Cu3N/MoS2 composite films was deposited on the substrate in a gas atmosphere of Ar and N2 at room temperature using a high degree of MoS2 target (99.99%) and a Cu target (99.99%).”
Comment 5: L200 “When the power of MoS2 was 4 W” – it can be reformulated, for clarity (it is not “the power of MoS2”, but the powder at which it was obtained).
Answer: First of all, we thank the reviewer very much for the comprehensive consideration to improve the accuracy of this article. We modified it as follows: “When the sputtering power of MoS2 was 4 W, the catalytic degradation rate of the composite films to methyl orange solution is faster and the degree of degradation was thorough.”
Comment 6：“Combined with the results of XRD spectrum analysis, S and Mo were complexed with Cu3N in the form of a MoS2 compound” – Please reformulate.
Answer: We are very sorry for our negligence of unclear statement, we added discussion as follows: “Combining the results of XRD spectrum analysis, we can know that MoS2 hasbeen successfully composited with Cu3N to form a composite film.”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this manuscript, Cu3N/MoS2 composite films were prepared by magnetron sputtering under different preparation parameters, and their photocatalytic properties were investigated. The photocatalytic degradation rate of methyl orange at 30 min was 98.3% because the MoS2 crystal in the films preferentially grew over the Cu3N crystal, thereby affecting the growth of the Cu3N crystal. The manuscript is well organized and contains interesting findings. However, I recommended a major revision of the article from its present form before it can be published in coatings. The main concerns are listed below.
In the present state, the introduction does not provide sufficient justification for this study. It should be rewritten to expound the research significance of the present work. It is better to provide a schematic representation for the experimental procedure. There are some flaws in the fitting of the XPS spectra. The authors should check. The authors should provide optical and photoluminescence studies. Trapping experiments are needed to explore the mechanism. The authors should check the loading effect of the catalyst for the degradation of dye. How to exclude the sensitization mechanism of MO for the present work? TOC is needed to sure the mineralization ratio. The influence of the medium must be studied. Standard deviations of results must be provided. The explanation of the photodegradation mechanism needs more clarification. The authors are advised to explain the mechanism with neat schematic representation. In the current state, there are more typographical errors and the language should be improved. Therefore, the authors are advised to recheck the whole manuscript for improving the language and structure carefully.
Author Response
Dear editor:
Thank you very much for your letter and advice. We have revised the manuscript, and would like to re-submit it for your consideration. We have addressed the comments raised by the reviewer and the amendment is highlighted in red in the revised manuscript. Points by point responses to the reviewer’s comments are listed below this letter.
We hope that the revised version of the manuscript is now acceptable for publication in your journal. And we are willing to pay for it.
We look forward to hearing from you soon.
Yours sincerely,
Xiao Jianrong
We would like to express our sincere thanks to reviewer for the constructive and positive comments.
Reviewer 2
Comment 1：In the present state, the introduction does not provide sufficient justification for this study. It should be rewritten to expound the research significance of the present work.
Answer: Considering the Reviewer’s suggestion, We have modified the article. The changes are as follows:
“Given that Cu3N and MoS2 have an adjustable bandgap structure, the photocatalytic performance can be improved by adjusting the size of the band gap, which is a promising photocatalytic material. Therefore, we conducted research in this area.”
“Although the Cu3N film alone has better photocatalytic performance, it still has some shortcomings, so researchers often dope or recombine Cu3N with other materials.”
“Among them, there are few reports on Cu3N and MoS2 composite films. ”
“In addition, compared to some preparation methods that require multiple steps, highly toxic precursors and pretreated substrates as growth templates, films prepared by magnetron sputtering have high purity, good uniformity and repeatability.”
Comment 2：It is better to provide a schematic representation for the experimental procedure.
Answer: Thank you very much for your suggestions, we have added in the article:
A schematic diagram of the experimental process is shown in Figure 1A Cu3N / MoS2 composite film was prepared on a silicon wafer by sputtering Cu target and MoS2 target simultaneously. Then, under the ultraviolet light irradiation, the composite orange film was used for catalytic degradation of the methyl orange solution. The illustration shows the principle of photocatalytic degradation of methyl orange solution by MoS2/Cu3N composite film.
Figure 1. Scheme of photocatalytic degradation of methyl orange solution by Cu3N/MoS2 composite films.
Comment 3：There are some flaws in the fitting of the XPS spectra. The authors should check.
Answer: Taking into account the reviewers' suggestions, we carefully checked the XPS spectrum and perfected the defects in the figure. The modified picture is shown below:
Figure 2. XPS diagram of Cu3N/MoS2 composite films: S 2p peak fitting
Comment 4：The authors should provide optical and photoluminescence studies. Trapping experiments are needed to explore the mechanism.
Answer: We performed optical studies on the films, supplemented the relevant data, and discussed the following in the article:
Figure 3a shows the UV-Vis transmission spectra of Cu3N/MoS2 composite films prepared with different power values. The film showed good transmittance in the infrared region. With the increase of the MoS2 compounding amount, the transmittance of the film gradually increases. According to the transmission spectrum curve of the film and the law of optical constants (1), the absorption coefficient of the film can be calculated as follows:
(1) Among them, T represents transmittance, and d represents film thickness. Using the Tauc Equation (2), the Eg of the film can be calculated:
(2) Where α, hv, and A represent the absorption coefficient, photon energy, and constant, respectively.
From the relationship curve of (αhv)2 and hv, calculate the band gap Eg by extrapolation, as shown in Figure 3b. The Eg range of Cu3N/MoS2 composite films is between 2.05-2.10 eV. As the power increases, the Eg of the composite film decreases first and then increases. When the deposition power is 4 W, the Eg of the film is the smallest, which is 2.05 eV. This may be because after MoS2 is recombined with Cu3N, Mo atoms replace Cu vacancies in the Cu3N lattice to act as donors, thus providing carriers to reduce the optical band gap; It is also possible that the film produced at a lower power has more defects. With the increase of power, the MoS2 content increases, and the optical band gap increases due to the hole filling effect in the valence band or the free carriers occupying the bottom of the conduction band.
Figure 3. UV-Vis transmission spectrum of Cu3N/MoS2 composite film
Taking into account the opinions of the reviewers, we tested the photoluminescence of the Cu3N/MoS2 composite film. The spectrum is shown below (Figure 4). Because the film is stored for a long time, it is polluted by impurities in the atmosphere and oxidized, resulting in poor photoluminescence test results of the film, which is difficult to cite in the article. We will continue to study the photoluminescence of this film in subsequent work.
Figure 4. Photoluminescence spectrum of Cu3N/MoS2 composite film
The reviewers have made a very good suggestion for the trapping experiment, but we are very sorry that we cannot conduct a trapping experiment, and we will follow this suggestion in future research work.
Comment 5：The authors should check the loading effect of the catalyst for the degradation of dye.
Answer: Thank you very much for your suggestion, but we mainly studied the influence of thin film sputtering power on dye degradation, and did not detect the loading effect of catalyst on dye degradation. We will follow this suggestion in future research work.
Comment 6：How to exclude the sensitization mechanism of MO for the present work?
Answer: I'm sorry we didn't give a detailed introduction in the article, here we make the following explanation: The experiment and transfer process of the degradation of methyl orange by the composite film are covered with tin foil to prevent the system from being exposed to visible light and causing dye sensitization.
Comment 7：TOC is needed to sure the mineralization ratio.
Answer: Many thanks to the reviewers for their suggestions. Due to time constraints, we cannot obtain TOC data. I will follow this suggestion in future research work.
Comment8：The influence of the medium must be studied.
Answer: Considering the reviewer's problem, we would like to explain as follows: Before performing the photocatalytic degradation reaction, we used a blank substrate as a control group to catalyze the degradation of methyl orange, and found that the substrate had no effect on the final conclusion of the experiment.
Comment 9：Standard deviations of results must be provided.
Answer: First of all, we thank the reviewer very much for the comprehensive consideration to improve the accuracy of this article. But since we have only done a set of experimental data, we cannot provide the standard deviation of the results.
Comment 10：The explanation of the photodegradation mechanism needs more clarification. The authors are advised to explain the mechanism with neat schematic representation.
Answer: Thank you very much for your suggestion. We have explained about the photodegradation mechanism in line 197 of the article: “When irradiated by UV light, the electrons of the low energy valence band of Cu3N absorbed enough energy to be excited and entered the high energy conduction band beyond the forbidden band. Negatively charged high activity electrons are produced on the conduction band, thereby leaving positively charged holes on the valence band and resulting in highly active photoelectron-hole pairs. After the electrons and holes are separated, they migrated to the Cu3N surface and combined with O2 and OH− in the solution to form O2- ions and •OH with strong oxidizing properties. These highly reactive particles undergo redox reactions with methyl orange, which in turn degrade methyl orange.”
Comment11：In the current state, there are more typographical errors and the language should be improved. Therefore, the authors are advised to recheck the whole manuscript for improving the language and structure carefully.
Answer: Thank you very much for your suggestions, we have checked the full text in detail and corrected typographical and grammatical errors. Your guidance makes the article more accurate, and we look forward to the article being published in your journal.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript can be accepted in the revised version.
