CuO/ZnO/CQDs@PAN Nanocomposites with Ternary Heterostructures for Enhancing Photocatalytic Performance

Round 1

Reviewer 1 Report

File attached.

Comments for author File: Comments.pdf

Author Response

Detailed Response to Review on " CuO/ZnO/CQDs@PAN nanocomposites with ternary heterostructures for enhancing photocatalytic performance" (Manuscript ID: catalysts-2089167)

 

First of all I should express my thanks to you for your careful reading, helpful comments.

 

This article systematically described the construction of heterojunction and the reasons for improved performance via materials characterization and performance tests. But some issues should be modified. There are comments and suggestions listed as follows:

 

1. There are serious errors in this article. The author needs to carefully modify the image annotation of the full text. For example, the first line of the first paragraph of Section 3.2 in the text is written "In FTIR patterns (Fig. 2i), while Figure 2(i) showed the XRD diagram. For another example, the eighth line of the first paragraph of Section 3.2 is written "In XRD spectra (Fig. 2j) ", but the picture of Fig. 2j does not exist in Figure 2.

Reply: Thank you for pointing out the errors. The labeling for FTIR and XRD patterns in Section 3.2 has been revised, and the picture labeling for the full text has been checked.

 

2. Please provide references for necessary data analysis. For example, "the absorption peaks at 2345 cm-1 , 1618 cm-1 and 1024 cm-1 , separately assigning to the stretching vibration of nitrile group C≡N 161 in PAN ……" in the first line of the first paragraph of Section 3.2, and "The peaks at 284.8 eV, 286.2 eV, and 288.4 eV in the XPS spectra of C 1s (Fig.3b) corresponded to sp2 (C=C/C-C), C-O, and carboxy COOR, respectively." in the second line of the first paragraph of Section 3.3.

Reply: Thank you for your valuable advice. The Refs. [38-42] have been provided for necessary data analysis in Sec. 3.2 and Sec. 3.3, and marked them using yellow background in the revised manuscript.

 

3. In the seventh line of the first paragraph of Section 3.2 is written "The stretching vibration of -OH was attributed to the broad absorption peak near 3426 cm-1 , which was more visible after loading CQDs." What is the role of hydroxyl at 3426 cm-1 ? In addition, the 3426 cm-1 peaks of the materials do not seem to differ significantly in the FTIR spectrum, especially compared with the -OH vibration peak of 3600-3500 cm-1 .

Reply: Thank you for your suggestion. The role of hydroxyl at 3426 cm^-1 is to characterize the existence of characteristic peaks of CQDs. The reason for why the 3426 cm^-1 peaks of the materials do not seem to differ significantly in the FTIR spectrum is that the content of CQDs is relatively low.

 

4. CZ@PAN and CZC@PAN showed good photodegradation efficiency. Section 3.5 mentioned that the test was conducted under the condition of "natural sunlight". Does this mean a full spectrum lamp or sunlight directly used outdoors? Have you compared the actual light intensity under natural sunlight and mercury lamp conditions? Have the respective quantum efficiencies been calculated?

Reply: Thank you for your suggestion. The test was conducted under the sunlight directly used outdoors. We did not compared the actual light intensity under natural sunlight and mercury lamp conditions, and did not calculated the respective quantum efficiencies. Your comments are very useful, and we will investigate them in further studies.

 

5. The results of the three-cycle tests seem to show a decreasing trend in performance. How many times can the catalyst be circulated?

Reply: Thank you for your valuable suggestion. We have not measured how many times the catalyst can be circulated. Theoretically, it can be circulated all the time. However, as multiple degradation cycles may lead to the adsorption of other impurities on the catalyst, the cycle test shows a downward trend in performance. In other reported articles, there are generally three-cycle tests, and the cycle degradation test also shows a downward trend in catalytic performance. We have added the illustration and marked them using yellow background in the revised manuscript.

 

6. In Section 3.3, Fig (e) mentioned "The results showed that the appearance of positive and negative slopes in the Mott-Schottky curve of CZC@PAN was a typical p-n junction feature." However, this work did not show the Mott-Schottky curve of CZ@PAN. The Mott-Schottky curve of CZ@PAN should be supplemented to prove the role of CQDs for the electronic energy band.

Reply: In our previous work [5], we have shown the Mott-Schottky curve for CZ@PAN, indicating that CZ@PAN is a typical p-n junction. In this work, the main purpose of the Mott-Schottky test is to prove that CZC@PAN is also a typical p-n junction feature. There, the main function of CQDs is to promote electron transfer, and they have no effect on the energy band structure. Compared with CZ@PAN, there is no new band structure, including a new bandgap formed in CZC@PAN. We have added the illustration and marked them using yellow background in the revised manuscript.

Reviewer 2 Report

(1)   The numbers of Fig. 2h and i are wrongly cited in the context.

(2)   The XRD peaks in the range of 20-30 degrees should be identified and indexed for the CZ@PAN and CZC@PAN heterojunctions.

(3)   All the XPS peaks in the survey spectra (Fig.3a) should be identified and indexed.

(4)   The conception of optical bandgap is not suitable for composite system, because the electrons from each component are still localized in respective energy bands, that is, there is not any new band structures including a new bandgap formed in composite systems. The related results and discussion should be corrected.

(5)   The band structures of the CuO/ZnO/CQDs@PAN heterojunctions lacks of evidences. The band structures of each component can be found in previous reports or obtained by applying some empirical formula to them, such as DOI: 10.1016/j.jcis.2022.01.073.

(6)   The role of each component in the CuO/ZnO/CQDs@PAN composite should be addressed and explained in detail by various control experiments. For this, the related references, including DOI: 10.1039/C2CE25540E, 10.1016/j.jcis.2022.01.073, and 10.1016/j.jphotochem.2017.05.012, can be referred to improve the quality of the manuscript.

Author Response

Detailed Response to Review on " CuO/ZnO/CQDs@PAN nanocomposites with ternary heterostructures for enhancing photocatalytic performance" (Manuscript ID: catalysts-2089167)

 

First of all I should express my thanks to you for your careful reading, helpful comments.

 

1. The numbers of Fig. 2h and i are wrongly cited in the context.

Reply: Thank you for pointing out the error. Fig. 2h and i have been modified in the diagram notes and context.

 

2. The XRD peaks in the range of 20-30 degrees should be identified and indexed for the CZ@PAN and CZC@PAN heterojunctions.

Reply: Thank you for your valuable advice. The XRD peaks in the range of 20-30 degrees have been identified and indexed for the CZ@PAN and CZC@PAN heterojunctions in Sec.3.2.

 

3. All the XPS peaks in the survey spectra (Fig.3a) should be identified and indexed.

Reply: Thank you for your valuable suggestion. The XPS peaks in the survey spectra (Fig.3a) have been identified and indexed.

 

4. The conception of optical bandgap is not suitable for composite system, because the electrons from each component are still localized in respective energy bands, that is, there is not any new band structures including a new bandgap formed in composite systems. The related results and discussion should be corrected.

Reply: Thank you very much for your valuable suggestion. The related results and discussion have been corrected in Sections 3.3 and 3.4, and marked them using yellow background in the revised manuscript.

 

5. The band structures of the CuO/ZnO/CQDs@PAN heterojunctions lacks of evidences. The band structures of each component can be found in previous reports or obtained by applying some empirical formula to them, such as DOI: 10.1016/j.jcis.2022.01.073.

Reply: Thank you for your suggestion. The Refs.[5, 14-18, 22, 44-46] have been provided for necessary evidences in Sec. 3.6, and marked them using yellow background in the revised manuscript.

 

6. The role of each component in the CuO/ZnO/CQDs@PAN composite should be addressed and explained in detail by various control experiments. For this, the related references, including DOI: 10.1039/C2CE25540E, 10.1016/j.jcis.2022.01.073, and 10.1016/j.jphotochem.2017.05.012, can be referred to improve the quality of the manuscript.

Reply: Thank you for your valuable advice. The combination of CuO and ZnO can form p-n heterojunctions, which can effectively separate the photogenerated electron-hole pairs to accelerate the photocatalytic reaction. And the up-converted photoluminescence (PL) behavior of CQDs can further extend the light utilization range of semiconductors, excite semiconductors to form more photogenerated electron-hole pairs, and enhance photoinduced electron transfer, thereby improving their photocatalytic degradation efficiency. Nanofiber membranes (NFMs) prepared by electrospinning technology have the advantages of large specific surface area, no secondary pollution and being easy to recycle. We have added the supplementary contents and marked them using yellow background in the revised manuscript. Besides, the related references [44, 46] have been added and referred in the manuscript.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments for author File: Comments.pdf

Author Response

Detailed Response to Review on " CuO/ZnO/CQDs@PAN nanocomposites with ternary heterostructures for enhancing photocatalytic performance" (Manuscript ID: catalysts-2089167)

 

First of all I should express my thanks to you for your careful reading, helpful comments.

 

The manuscript entitled “CuO/ZnO/CQDs@PAN nanocomposites with ternary heterostructures for enhancing photocatalytic performance” reported the ternary composite systems of CuO-ZnO-CQDs over PAN for photocatalytic degradation of MB dye solution under sunlight. This research claimed to enhance photocatalytic activity for MB degradation with CQDs in the composite. However, activities of the catalysts are not at par with traditional photocatalysts such as Degussa P25. Moreover, this study lacks experimental proof such as how this ternary system helps in activity enhancement. Introduction part need to be re reviewed for thorough study of literature.

 

1. Why you choose the specific composite system? Although, its photocatalytic activity is even lower than the previously reported articles (Tuncel et al. Catal. Today 2019, 328, 149-156; Sakib et al. Journal of Composites Science 2019, 3).

Reply: Thank you for your valuable advice. The combination of CuO and ZnO can form p-n heterojunctions, which can effectively separate the photogenerated electron-hole pairs to accelerate the photocatalytic reaction. And the up-converted photoluminescence (PL) behavior of CQDs can further extend the light utilization range of semiconductors, excite semiconductors to form more photogenerated electron-hole pairs, and enhance photoinduced electron transfer, thereby improving their photocatalytic degradation efficiency. Nanofiber membranes (NFMs) prepared by electrospinning technology have the advantages of large specific surface area, no secondary pollution and being easy to recycle. Therefore, we choose the specific composite system (CZC@PAN). We have compared CZC@PAN with other ZnO-based nanophotocatalysts, as shown in Table 1. It was found that the nanoparticle-like photocatalysts had better degradation effects [29,30], but they are not easy to recover and reuse. We have added the supplementary contents in Introduction, and marked them using yellow background in the revised manuscript.  

 

2. In XRD, there is a broad hump around 2θ = ~14° and sharp peak at 2θ = 7° (assigned to C-/Znacetate). Is it an impurity peak (2θ = ~14°)? authors should identify this. Why there are still peaks for C-/Zn-acetate precursors in the sample?

Reply: Thank you for your advice. It is an impurity peak (2θ= ~14°). The reason why there are still peaks for Cu/Zn-acetate precursors in the sample is that the Zn(Ac)₂ and Cu(Ac)₂ in the NFMs were not fully decomposed into ZnO and CuO due to the low heat treatment temperature (150 °C). We have added the illustrations and marked them using yellow background in the revised manuscript.

 

3. The role CQDs is not explained in detail without any experimental evidence, just hypothesis is given that it helps in transfer of electrons.

Reply: Thank you for pointing out this problem. The effect of CQDs has been reported in previous articles [15-20]. In addition, we demonstrated that it can effectively block the recombination of photogenerated electron hole pairs by UV-Vis and PL tests, and further revealed that the addition of CQDs can improve the photocatalytic activity through the photocatalytic degradation tests.

 

4. Only PL is utilized to show improved catalytic efficiency of ternary catalyst based on charge carrier recombination dynamics. What about other characterizations, like charge transfer resistance (EIS), lifetime of photogenerated charge carriers (time-decay PL /TCSPC) or photocurrent densities. It will help you understand the mechanism and role of different catalysts in this composite system. Often, it is observed that catalytic activity is an interplay of several factors not only one (doi.org/10.1039/D1MA00304F Mater. Adv., 2021, 2, 4832).

Reply: Thank you for your valuable suggestion. But we are sorry that some laboratories are not open due to the impact of the coronavirus epidemic situation, causing that experiments couldn’t be conducted. Your comments are very useful, and we will further investigate them in future studies.

 

5. Authors, should construct a band energy level diagram to show the mechanism of photocatalyst by calculating the CBM and VBM of n-type and p-type semiconductor catalysts, respectively by calculating their flat-band potential (Mott-Schottky plots) and bandgap values. There is no relevance of showing mott-schottky plot of ternary catalyst.

Reply: Thank you very much for your valuable suggestion. The main purpose of the Mott-Schottky test is to prove that CZC@PAN is a typical p-n junction feature. But we are sorry that some laboratories are not open due to the impact of the coronavirus epidemic situation, causing that experiments couldn’t be conducted. Your comments are very useful, and we will investigate them in future studies.  

 

6. There are some typo errors in the manuscript, like in Fig. 1 both the particles (orange and blue) marked as ZnO, thus a thorough check on this manuscript is needed.

Reply: Thank you for pointing out the errors. The typo error in Fig.1 has been corrected, and the other errors in the manuscript have been checked and revised.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Authors improved the manuscript significantly, but stillsome questions need to be answered.

1.  The 2θ = ~14° (broad peak) is not identified in xrd of CZ-NPs-CNFMs. Is it an impurity peak (2θ = ~14°)?  

2. In the Fig. 5b, authors should atleast show the band positions of CB and VB for CuO and ZnO as well as redox potentials for ·O^2– and ·OH radical generation w.r.t. N.H.E. as band potentials does not change in composite systems and for CuO and ZnO, its available in literature. 

Author Response

Detailed Response to Review on " CuO/ZnO/CQDs@PAN nanocomposites with ternary heterostructures for enhancing photocatalytic performance" (Manuscript ID: catalysts-2089167)

 

First of all I should express my thanks to you for your careful reading, helpful comments.

 

Authors improved the manuscript significantly, but stillsome questions need to be answered.

 

1. The 2θ = ~14° (broad peak) is not identified in xrd of CZ-NPs-CNFMs. Is it an impurity peak (2θ = ~14°)?

Reply: Yes, it is an impurity peak (2θ= ~14°).

 

2. In the Fig. 5b, authors should at least show the band positions of CB and VB for CuO and ZnO as well as redox potentials for ·O^2- and ·OH radical generation w.r.t. N.H.E. as band potentials does not change in composite systems and for CuO and ZnO, its available in literature.

Reply: Thank you for your valuable suggestion. According to Refs. [30, 47], we have shown the band positions of CB and VB for CuO and ZnO as well as redox potentials for·O^2- and·OH radical generation w.r.t. N.H.E. in Fig. 5b. The related illustrations and Ref.[47] have been added in Section 3.6 and marked with blue background in the revised manuscript.

Author Response File: Author Response.pdf