Visible Light Driven Spherical CuBi₂O₄ with Surface Oxygen Vacancy Enhanced Photocatalytic Activity: Catalyst Fabrication, Performance, and Reaction Mechanism

Round 1

Reviewer 1 Report

The paper of Xin Zhong et al on visible light-driven photocatalytic degradation of RhB over spherical CuBi2O4 with surface oxygen vacancy is not, in the present form, acceptable for publication in Catalysts.

The document must be proofread and corrected; among many :

Line 36: Could the authors define “SR reaction” and line 38 “SR-photo process”?

Line 40: “peroxymonosulfate (PMS)” should be potassium peroxymonosulfate, see also line 109. The term “oxone” could be also must more familiar for most chemists.

Line 48: “the transferring of transition metal ions.”?

Lines 198-199: “as well as using in single presence of PMS or LED light for nearly no formation of reactive free radicals.” ?

Lines 205-206: “with the the introduce of surface oxygen vacancy ”?

Lines 206-207: Can a “synergy effect” could be “accelerated”?

Line 207: “The results might attribute to to the”

….

 

However, the main problem is the photocatalyst. Why PMS must be used and what is the molar ratio between RhB and potassium peroxymonosulfate? In section 2.3, we could only found that a “certain amount of photocatalyst was added” (line 107) and “PMS was added” (line 109)!! In the caption of figure 6, one can read that 20 mg/L of RhB was used for a concentration of PMS of 0.4mM. Again did oxone was used in large excess? I could calculated that a ratio PMS/RhB of approximately 10 was used.

As such the authors should argue why we should use this new photocatalytic material since several materials already achieved the visible-light photocatalytic degradation of RhB (with~95–99.5% degradation) within 30 min without using extra oxidizing agent like PMS (see for example Journal of Colloid and Interface Science 561 (2020) 71–82; DOI : 10.1016/j.jcis.2019.11.118).

Using the same concentration of RhB, the photocatalytic degradation of RhB over NaBiO3 under visible light irradiation was almost completely decolorized in 30 min (See J. Phys. Chem. A 2009, 113, 37, 10024–10032; DOI : 10.1021/jp905173e).

The use of an extra oxidizing agent (10 eq of PMS) doesn't make this material valuable or, authors should compare or argue with known photocatalysts.

Author Response

Please see the attachment.

Dear Reviewers:

Thank you for your comments concerning our manuscript entitled ‘Visible light driven spherical CuBi₂O₄ with surface oxygen vacancy enhanced photocatalytic activity: catalyst fabrication, performance and reaction mechanism’ (No.: catalysts-890665). Those comments are valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research.

We have studied comments carefully and have made revisions which we hope meet with your approval. Revised text in the manuscript are marked in red in the revised manuscript. The main corrections in the paper and the responds to the reviewer’s comments are as flowing.

Best regards.

Xin Zhong

E-mail: [email protected]

 

Please find the following response to the comments of referees.

Response to the reviewer’s comments. (Reviewer's comments marked in blue)

 

Response to Reviewer 1 Comments

Point 1 The paper of Xin Zhong et al on visible light-driven photocatalytic degradation of RhB over spherical CuBi₂O₄ with surface oxygen vacancy is not, in the present form, acceptable for publication in Catalysts.

Response: Thanks for your kind suggestion.

We have tried our best to modified this manuscript. We carefully checked and revised the mistakes and misunderstandings in the revised manuscript. And the revised manuscript has been reviewed by native English speaker and revised to improve readability. The changes are marked red in the revised manuscript. We appreciate reviewers' warm advice earnestly, and hope the correction will meet with approval.

Point 2 The document must be proofread and corrected; among many:

1. Line 36: Could the authors define “SR reaction” and line 38 “SR-photo process”?

Response: Thanks for your kind suggestion.

We are sorry for the unclear writing of ‘SR reaction’ and ‘SR-photo process’. ‘SR reaction’ is short for sulfate radical-based Fenton reaction, which has been corrected as sulfate radical-based advance oxidation processes (SR AOPs) in line 33. ‘SR-photo process’ is short for the heterogeneous sulfate radicals-based reaction coupled with light irradiation, which is similar to the photo-Fenton reaction but using PMS instead of H₂O₂ in the reaction. In the revised manuscript, it has been changed as ‘heterogeneous SR-photo-Fenton like process’ in line 39. We have carefully revised this unclear part and marked in red in the revised manuscript. Thank you very much for your comments and suggestions.

2. Line 40: “peroxymonosulfate (PMS)” should be potassium peroxymonosulfate, see also line 109. The term “oxone” could be also must more familiar for most chemists.

Response: Thanks for your kind suggestion.

As reviewer suggested that potassium peroxymonosulfate (PMS) is applied in this paper. PMS is short for potassium peroxymonosulfate, which widely used as an oxidizing agent. And the trade name Oxone is also known and studied by many researchers. We modified PMS as ‘potassium peroxymonosulfate (PMS, trade name Oxone)’ in line 37. The changes are marked red in the revised version. Special thanks to you for your good comments.

3. Line 48: “the transferring of transition metal ions.”?

Response: Thanks for your kind suggestion.

We are sorry for the unclear writing of this sentence. The sentence has been corrected and marked red in the revised manuscript, which corrected as ‘Thus, it could improve the reduction between the high valence state and the low valence state of metal ions, improving the cycling of transition metal ions.’ in line 51. Special thanks to you for your good comments.

4. Lines 198-199: “as well as using in single presence of PMS or LED light for nearly no formation of reactive free radicals.” ?

Response: Thanks for your kind suggestion.

We are sorry for the unclear writing of this sentence. The sentence was corrected as ‘And negligible removal of RhB was obtained by using PMS alone and using LED light alone, due to hardly production of reactive free radicals.’ in line 210. The texts are marked red in the revised manuscript. Thank you very much for your comments and suggestions.

5. Lines 205-206: “with the the introduce of surface oxygen vacancy ”?

Response: Thanks for your kind suggestion.

We are sorry for the incorrect writing of this sentence. The surplus word ‘the’ was deleted in line 210. The sentence was corrected as ‘with the introduce of surface oxygen vacancy’. We have carefully checked the whole manuscript to avoid the same mistakes. Thank you very much for your comments and suggestions.

6. Lines 206-207: Can a “synergy effect” could be “accelerated”?

Response: Thanks for your kind suggestion.

As reviewer suggested that synergism effect was obtained in the reaction. The sentence was corrected as ‘with the introduction of surface oxygen vacancy, it was observed there was the synergy effect between heterogeneous SR-Fenton like reaction and photocatalysis, which improved the degradation activity of RhB’. Thank you very much for your comments and suggestions.

7. Line 207: “The results might attribute to to the”

Response: Thanks for your kind suggestion.

We are sorry for the incorrect writing of this sentence. The surplus word ‘to’ was deleted. The sentence was corrected as ‘The results might attribute to’ in line 220. We have carefully checked the whole manuscript to avoid the same mistakes. Thank you very much for your comments and suggestions.

Point 3 However, the main problem is the photocatalyst. Why PMS must be used and what is the molar ratio between RhB and potassium peroxymonosulfate? In section 2.3, we could only found that a “certain amount of photocatalyst was added” (line 107) and “PMS was added” (line 109)!! In the caption of figure 6, one can read that 20 mg/L of RhB was used for a concentration of PMS of 0.4mM. Again did oxone was used in large excess? I could calculated that a ratio PMS/RhB of approximately 10 was used.

Response: Thanks for your kind suggestion.

PMS was used in the photocatalysis reaction to accelerate the reaction rate, considered as an environment-friendly oxidant. In this paper, as can be seen that the rate constant is 0.0064 h^-1, 0.0126 h^-1 and 0.0304 h^-1 for photocatalysis, heterogeneous SR-Fenton like reaction and heterogeneous SR-photo-Fenton like reaction. As the photocatalysis reaction always suffered the long reaction time and high power of light source (usually 300-500 W Xe lamp in earlier research), PMS is not only an oxidant activated by the catalyst to generate reactive radicals, but also an electron acceptor that can decline the recombination of photogenerated electrons and holes, leading to better degradation efficiency. It was verified that there was synergistic effect between the photocatalysis process and the heterogeneous SR-Fenton like reaction.

We are sorry for the unclear writing of the amount of catalyst and PMS in the original manuscript. In a typical experiment, the catalyst addition was 0.1 g, and the addition of PMS was 0.049 g. We have carefully modified the section 2.3 in line 113 and line 115.

In this paper, as can be seen in Fig.6, the concentration of RhB was 20 mg/L (0.0417 mM) and the optimal PMS concentration was 0.4 mM, where the molar ratio of PMS (oxidant) to target pollutant (RhB) was 9.582, nearly approach to 10:1. It was obtained that the degradation efficiency of photocatalysis, heterogeneous SR-Fenton like reaction and heterogeneous SR-photo-Fenton like reaction was 29.4%, 56.8%, 87.9%, respectively. With the presence of PMS, the degradation efficiency was significantly enhanced. As the increment of PMS concentration, the degradation efficiency was enhanced, where 39.1% for addition of 0.1 mM PMS and 77.9% for addition of 0.2 mM PMS. However, the degradation efficiency of RhB was not further increased when the PMS concentration was higher than 0.4 mM. The results showed that the degradation efficiency got higher with higher PMS concentration, indicating the addition of PMS was not enough for the complete degradation. Thus, overdosed PMS will deplete the reactive radicals, resulting in declined removal efficiency of RhB.

As reviewer mentioned, it is necessary to find the theoretical stoichiometry of PMS concentration for the complete mineralization of 0.0417 mM RhB. To degrade 1 mol RhB, 73 mol PMS was needed to get the complete mineralization.

C₂₈H₃₁ClN₂O₃+ 73HSO₅^-→ 28CO₂ + 76H⁺ +14H₂O + Cl^- + 2NO₃^- + 73SO₄^2-      (1)

In the CuBi₂O₄/PMS/LED process, the theoretical stoichiometry of PMS was 3.044 mM which was calculated for the complete mineralization of 0.0417 mM RhB. Due to above consideration, the optimal concentration of PMS (0.4 mM) was lower than the theoretical stoichiometry concentration which further illustrated the incomplete TOC removal of RhB. We tried our best to improve the manuscript. Your kind suggestions helped us to improve this manuscript a lot.

Point 4 As such the authors should argue why we should use this new photocatalytic material since several materials already achieved the visible-light photocatalytic degradation of RhB (with~95–99.5% degradation) within 30 min without using extra oxidizing agent like PMS (see for example Journal of Colloid and Interface Science 561 (2020) 71–82; DOI: 10.1016/j.jcis.2019.11.118). Using the same concentration of RhB, the photocatalytic degradation of RhB over NaBiO3 under visible light irradiation was almost completely decolorized in 30 min (See J. Phys. Chem. A 2009, 113, 37, 10024–10032; DOI : 10.1021/jp905173e). The use of an extra oxidizing agent (10 eq of PMS) doesn't make this material valuable or, authors should compare or argue with known photocatalysts.

Response: Thanks for your kind suggestion.

To the best of our knowledge, there was little study on the application of LED lamp (460-470 nm) as the light source by using CuBi₂O₄-OVs materials in the photocatalytic process with the introduction of PMS. However, the light sources were mostly attributed to 300-500 W Xenon lamp or gold halide/ tungsten lamp for the application of photocatalysis or heterogenous photo-Fenton like process in the previous reports ^[5]. It was proved that there was synergistic effect between the catalyst/PMS and catalyst/visible light. The degradation efficiency of the target pollutant was assumed to be further improved. On the other hand, to achieve better degradation efficiency, large amount of photocatalyst was utilized in the systems ^[2]. In this work, the focus is on the application of LED light in the heterogeneous PMS activation system. For comparison, photocatalytic efficiency of pure CuBi₂O₄ catalyst in the absence and presence of PMS/PS was investigated in Table 1. The photocatalytic performance of the pure CuBi₂O₄ in the absence of PMS was not satisfied, while in the presence of PMS/PS, the photocatalytic activity was improved.

To evaluate the photocatalytic activity of CuBi₂O₄-OVs in this work, in the heterogeneous SR-Fenton like system, the RhB obtained 56.8% degradation efficiency, which is in accordance with the Wang’ work ^[1], which used pure CuBi₂O₄ as catalyst to reach 50.5% removal efficiency of RhB by using 300 W xenon lamp. It was studied that the photocatalytic activity of pure CuBi₂O₄ catalyst was always suffered rapidly recombined photo-generated charge, leading to poor degradation efficiency or a long reaction time. Many researchers studied the doping strategy and the heterojunction with other semiconductors to improve the photocatalytic activity of CuBi₂O₄. In this work, a 30 W LED lamp was employed in the heterogeneous SR-photo-Fenton like reaction by using CuBi₂O₄-OVs as catalyst, which was improved with the presence of surface oxygen vacancy on the catalyst through one-pot hydrothermal method. Moreover, the CuBi₂O₄-OVs was also employed for the degradation of other organic contaminants in the heterogeneous SR-photo-Fenton like reaction, such as Orange II, methyl red (MR), ciprofloxacin (CIP) and levofloxacin (LVF), the results were showed in Fig.8b. The degradation efficiency of Orange II, methyl red, CIP and LVF was 85.1%, 83.7% 10.4% and 16.2%, respectively. The results showed that the heterogeneous SR-photo-Fenton like reaction was effective for mostly common organic pollutants, especially for the removal of dyes.

We conclude the novelty of this manuscript as i) The CuBi₂O₄ with surface oxygen vacancy with higher utilization efficiency in the heterogeneous SR-photo-Fenton like system; ii) the degradation mechanism of RhB was put forward based on the experimental results in the CuBi₂O₄-OVs/PMS/LED process with various experimental conditions. We tried our best to improve the manuscript and made some changes in the manuscript. Your kind suggestions helped us to improve this manuscript a lot.

Table 1 Studies on of Bi-based catalysts in the photocatalytic reactions*

No.	Systems	Light source	Target  pollutant	Reaction time and efficiency	Operation parameters
1	Vis+CuBi2O4(pure) +PMS	300W xenon lamp	RhB 25 mg/L	50.5% 180 min	[RhB]=25 mg/L [PMS]=0.65 mM [catalyst]=0.8g/L pH=3.4
2	Bi1.09Sb0.91S3+VIS	CFL 60 W	RhB 10 ppm	97.71% 30 min	[RhB]=10 ppm [catalyst]=5 g/L
3	CuBi2O4+Vis	500W xenon lamp	CIP 10ppm	26% 180min	[CIP]=10 ppm [catalyst]=1 g/L
4	PMS/CuBi2O4/VL	300W xenon lamp	TC 50 mg/L	Nearly complete removal 60 min	[PMS]= 0.125 mg/mL [catalyst]=0.5 g/L
5	NaBiO3+Vis	750W xenon lamp	RhB 20 mg/L	100% 30min	[catalyst]=1.0 g/L
6	CuBi2O4+Vis+PMS	35W xenon lamp	Ceftiofur 5 mg/L	Nearly 50% 40min	[PMS] =0.4g/L [catalyst]=0.3 g/L pH=5.8
7	CuBi2O4+Vis	300 W xenon lamp	Tetracycline 20 mg/L	9.2%  60min	[catalyst]=0.3 g/L
8	CuBi2O4+Vis	500 W xenon lamp	Norfloxacin 10 mg/L	11% 60 min	[catalyst]=1.0 g/L
9	CuBi2O4+Vis	300 W xenon lamp	Tetracycline 20 mg/L	36% 120min	[catalyst]=1.0 g/L
10	CuBi2O4+Vis	300 W xenon lamp	Diclofenac 10 mg/L	67.12% 120 min	[catalyst]=0.5 g/L
11	CuBi2O4+Vis+PS	50 W LED lamp	RhB 1*10-5 M (4.79 mg/L)	Nearly 80% 300 min	[catalyst]=0.5 g/L [PS]=1.48 mM
This work	CuBi2O4+Vis+PMS	30 W LED lamp	RhB 20 mg/L	87.9% 60 min	[catalyst]=0.5 g/L [PS]=0.4 mM Neutral pH

*only photocatalytic activity of pure CuBi₂O₄ was discussed in this table.

 

[1] Y. Wang, C. Liu, Y. Zhang, W. Meng, B. Yu, S. Pu, D. Yuan, Fei Qi, B. Xue, W. Chu. Sulfate radical-based photo-Fenton reaction derived by CuBi₂O₄ and its composites with α-Bi₂O₃ under visible light irradiation: Catalyst fabrication, performance and reaction mechanism. Applied Catalysis B: Environmental, 2018, 235, 264-273.

[2] L. Dashairya, A. Mehta, P. Saha, S. Basu. Visible-light-induced enhanced photocatalytic degradation of Rhodamine-B dye using BixSb2-xS3 solid-solution photocatalysts. Journal of Colloid and Interface Science, 2020, 561, 71-82.

[3] Z. Li, M. Chen, Q. Zhang, D. Tao. Mechanochemical synthesis of a Z-scheme Bi₂WO₆/CuBi₂O₄ heterojunction and its visible-light photocatalytic degradation of ciprofloxacin. Journal of Alloys and Compounds, 2020, 845, 156291.

[4] J. Zhang, C. Zhai, W. Zhao, Y. Chen, R. Yin, L. Zeng, M. Zhu. Insight into combining visible-light photocatalysis with transformation of dual metal ions for enhancing peroxymonosulfate activation over dibismuth copper oxide. Chemical Engineering Journal, 2020, 397, 125310.

[5] K. Yu, S. Yang, H. He, C. Sun, C. Gu, Y. Ju. Visible Light-Driven Photocatalytic Degradation of Rhodamine B over NaBiO₃: Pathways and Mechanism. J. Phys. Chem. A 2009, 113, 10024-10032.

[6] H. Zhang, L. Nengzi, X. Li, Z. Wang, B. Li, L. Liu, X, Cheng. Construction of CuBi₂O₄/MnO₂ composite as Z-scheme photoactivator of peroxymonosulfate for degradation of antibiotics. Chemical Engineering Journal, 2020, 386, 124011.

[7] W. Shi, M. Li, X. Huang, H. Ren, F. Guo, Y. Tang, C. Lu. Construction of CuBi₂O₄/Bi₂MoO₆ p-n heterojunction with nanosheets-onmicrorods structure for improved photocatalytic activity towards broadspectrum antibiotics degradation. Chemical Engineering Journal, 394, 125009.

[8] X. Zhang, X. Wang, J. Chai, S. Xue, R. Wang, L. Jiang, J. Wang, Z. Zhang, D.D. Dionysiou. Construction of novel symmetric double Z-scheme BiFeO₃/CuBi₂O₄/BaTiO₃ photocatalyst with enhanced solar-light-driven photocatalytic performance for degradation of norfloxacin. Applied Catalysis B: Environmental, 2020,272,119017.

[9] L. Wang, G. Yang, D. Wang, C. Lu, W. Guan, Y. Li, J. Deng, J. Crittenden. Fabrication of the flower-flake-like CuBi₂O₄/Bi₂WO₆ heterostructure as efficient visible-light driven photocatalysts: Performance, kinetics and mechanism insight. Applied Surface Science, 2019, 495, 143521.

[10] X. Chen, Y. Dai, J. Guo. Hydrothermal synthesis of well-distributed spherical CuBi₂O₄ with enhanced photocatalytic activity under visible light irradiation. Materials Letters, 2015, 161, 251-254.

 

We tried our best to improve the manuscript and made some changes in the manuscript. Your kind suggestions helped us to improve this manuscript a lot. We appreciate for reviewer’s warm advice earnestly, and hope that the correction will meet with approval.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript reported by X. Zhong and coworkers reports on the preparation and detailed characterization of a mixed oxide based mateterial, CuBi₂O₄, which exhibits chemical degradation ability in the heterogeneous sulfate radical based reaction. In general, I think authors have performed large effort to fully characterize the material and they bring some important conclusions on the work, so my feeling is that the work may be accepted for publication. However, the manuscript shows a worrying level of mistakes and a poor redaction, which should be improved before the work is eventually accepted. I raise my main concerns to authors in the following points:

• In the first paragraph of the introduction, authors list some different kinds of materials acting as photocatalysts, involving MIL-53(Fe) and some inorganic materials. It could fine if authors could say some words about the different nature of materials, taking into account that the first one is an hybrid metal-organic framework (MOF) whereas the rest are inorganic oxides.
• The manuscript is not well typed and English should be extensively improved. This is clearly observed in the second paragraph of the introduction with sentences as the following: "However, the photocatalysis performance of CuBi₂O₄ was always suffered rapidly recombined photo-generated charge, leading to the limited application of CuBi₂O₄." in line 60-61.
• Although authors provide extensive characterization with several techniques. However, in the paragraph of line 126 this idea is not so clear. Therefore, I would encourage authors to give further explanation on the synthesis as to know whether there is any control between the crystallization of CuBi₂O₄-OV phase vs CuBi₂O₄.
• In Figure 1, an inset seems to accidentally out from the figure. The rendering should be improved and those results should be explained in the footnote.
• In the images shown of HRTEM shown in Figure 2, it seems to me that one needs to imagine the lattice spacing. It is completely imposible to estimate based on the images given in the actual size, the pixels cannot be well distinguished at such quality.
• As dictated by IUPAC rules on nomenclature, the radical species must be written in the following form: "SO₄^–". Please correct this issue.
• In the analysis of reaction conditions, authors state that, when TBA and MeOH are added to the reaction mixture, the reaction is slowed down. It would be great if they could better explain this effect in the paragraph of page 11.
• Here I have typed some of the mistakes previously mentioned, although there are much more and cannot be listed in this document:
  • In the abstract, please, type "could be attributed" instead of "could attribute" and What do authors mean by "amount” in line 19?
  • In line 29, please rewrite "...problem has attracted more and more attention..." instead of "problem has been paid more and more attention..."
  • In line 31, type "brought" instead of "attracted"
  • In line 40, please rewrite the following sentence as: "leading to the formation of a series of reactive radicals".
  • The sentence starting by: "In heterogeneous SR-photo process, PMS..." in line 43 is not correctly typed and is not well understood, please correct it.
  • In line 52, please retype it as: "...catalysts have received more attention..."
  • In line 86, please rewrite the excerpt: "Then, both solutions were mixed..."
  • In line 89, in the synthetic details, authors state that "CuBi2O4 was also prepared without glucose." Why do they do in this way? Some explanation would be welcome in this sentence.
  • In line 111, TOC is mentioned for the first time without specifying its meaning.
  • In line 147, please type "introduction" instead of "introduce".
  • Please, correct the double "the" in line 205.
  • In line 207: "The results might be attributed to..."
  • In line 316, the word "hardly" would be better typed as "difficult"

Author Response

Please see the attachment.

Dear Reviewers:

Thank you for your comments concerning our manuscript entitled ‘Visible light driven spherical CuBi₂O₄ with surface oxygen vacancy enhanced photocatalytic activity: catalyst fabrication, performance and reaction mechanism’ (No.: catalysts-890665). Those comments are valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research.

We have studied comments carefully and have made revisions which we hope meet with your approval. Revised text in the manuscript are marked in red in the revised manuscript. The main corrections in the paper and the responds to the reviewer’s comments are as flowing.

Best regards.

Xin Zhong

E-mail: [email protected]

 

Please find the following response to the comments of referees.

Response to the reviewer’s comments. (Reviewer's comments marked in blue)

 

Response to Reviewer 2 Comments

The manuscript reported by X. Zhong and coworkers reports on the preparation and detailed characterization of a mixed oxide based mateterial, CuBi2O4, which exhibits chemical degradation ability in the heterogeneous sulfate radical based reaction. In general, I think authors have performed large effort to fully characterize the material and they bring some important conclusions on the work, so my feeling is that the work may be accepted for publication. However, the manuscript shows a worrying level of mistakes and a poor redaction, which should be improved before the work is eventually accepted. I raise my main concerns to authors in the following points:

Point 1 In the first paragraph of the introduction, authors list some different kinds of materials acting as photocatalysts, involving MIL-53(Fe) and some inorganic materials. It could fine if authors could say some words about the different nature of materials, taking into account that the first one is an hybrid metal-organic framework (MOF) whereas the rest are inorganic oxides.

Response: Thanks for your kind suggestion.

As reviewer suggested that it was necessary to illustrated the different nature of these mentioned materials. The sentence was modified as ‘Many kinds of heterogeneous catalysts had been investigated and applied in the heterogeneous SR-photo-Fenton like process, such as hybrid metal-organic framework-MIL-53(Fe), inorganic oxide heterojunction-CuBi₂O₄-Bi₂O₃, metal oxides-Co₃O₄, perovskite structure-BiFeO₃.’ in line 45. The texts are marked red in the revised manuscript. Thanks again for your suggestions.

Point 2 The manuscript is not well typed and English should be extensively improved. This is clearly observed in the second paragraph of the introduction with sentences as the following: "However, the photocatalysis performance of CuBi2O4 was always suffered rapidly recombined photo-generated charge, leading to the limited application of CuBi2O4." in line 60-61.

Response: Thanks for your kind suggestion.

We are sorry for the incorrect writing of this sentence. The sentence was corrected as ‘However, the photocatalytic performance was poor by single usage of CuBi₂O₄, which suffered quick recombination of photogenerated holes and electrons.’ in line 66. The texts are marked red. We have carefully checked the whole manuscript to avoid the similar mistakes. And the revised manuscript has been reviewed by native English speaker and revised to improve readability. Thank you very much for your comments and suggestions.

Point 3 Although authors provide extensive characterization with several techniques. However, in the paragraph of line 126 this idea is not so clear. Therefore, I would encourage authors to give further explanation on the synthesis as to know whether there is any control between the crystallization of CuBi2O4-OV phase vs CuBi2O4.

Response: Thanks for your kind suggestion.

As reviewer suggested, we changed the image of Fig.1b in order to get clear images for CuBi₂O₄-OVs. We also specified the different morphology between CuBi₂O₄-OVs and CuBi₂O₄ due to the difference in the synthesis procedure. The texts were in line 130. ‘Fig.1 presented that the CuBi₂O₄ sample was composed of a sphere-like morphology which was made of nanocrystals of 20-40 nm thickness. However, it was also observed that large bulks were presented on the surface with agglomeration, owing to the anisotropic growth of crystals on the surface. The diameter of the sphere was in the width of ~400 nm. On the other hand, the morphology of CuBi₂O₄-OVs synthesized after the addition of glucose showed smaller diameter of ~350 nm with a wool-ball like morphology, which was different with the CuBi₂O₄. The results can be attributed to the reductant of glucose, which prevent growth of crystallization, resulting in the decreased crystallinity of CuBi₂O₄ sample. ’ Thank you very much for your comments and suggestions.

Point 4 In Figure 1, an inset seems to accidentally out from the figure. The rendering should be improved and those results should be explained in the footnote.

Response: Thanks for your kind suggestion.

As reviewer suggested, the inset paragraph was put inside the Figure 1b and the rendering was carefully checked. We carefully revised the whole manuscript to avoid the similar problems. Thank you very much for your comments and suggestions.

Point 5 In the images shown of HRTEM shown in Figure 2, it seems to me that one needs to imagine the lattice spacing. It is completely imposible to estimate based on the images given in the actual size, the pixels cannot be well distinguished at such quality.

Response: Thanks for your kind suggestion.

As reviewer suggested, we changed the images of HRTEM in Figure 2 to get clear photograph. The inset image was the magnified image in the circle to get clear lattice spacing. We carefully modified these HRTEM images to show distinct lattice structure. We tried our best to obtain clear pixels and avoid the distortion in these images.

Point 6 As dictated by IUPAC rules on nomenclature, the radical species must be written in the following form: "SO4–". Please correct this issue.

Response: Thanks for your kind suggestion.

As reviewer suggested, the sulfate radical was correct in the form of ‘SO₄^·-’. We appreciate your kind reminding and checked the whole part in the revised manuscript. The modified texts were marked red. Thank you very much for your comments and suggestions.

Point 7 In the analysis of reaction conditions, authors state that, when TBA and MeOH are added to the reaction mixture, the reaction is slowed down. It would be great if they could better explain this effect in the paragraph of page 11.

Response: Thanks for your kind suggestion.

As reviewer suggested, the effect of scavenging chemicals was illustrated in line 311. The modified texts were in the following, and marked red in the revised manuscript.

‘In this case, TBA was usually considered as the trapping agent of OH. Methanol (MeOH) could react with sulfate and hydroxyl radicals in the meantime, which was used as the trapping agent of sulfate radicals and hydroxyl radicals. It can be seen in Fig.9a that the degradation efficiency of RhB was 79.5% and 70.6%, which slightly decreased in the presence of TBA and MeOH compared to the control experiment. Herein, it was proposed that both of sulfate radicals and hydroxyl radicals were formed in the reaction. However, the two free radicals took a little contribution for the reaction, implying the two scavenging chemicals took a moderate inhibition in the reaction. ’ Thank you very much for your comments and suggestions.

Point 8 Here I have typed some of the mistakes previously mentioned, although there are much more and cannot be listed in this document:

Response: Thanks for your kind suggestion.

As reviewer mentioned, it is true that there were many mistakes in the original version. We are sorry for these mistakes and unclear writing. We have already checked the whole manuscript to avoid these problems in order to make the manuscript better redaction.

• In the abstract, please, type "could be attributed" instead of "could attribute" and What do authors mean by "amount” in line 19?

In the abstract, the sentence was corrected as ‘In the interface reaction, the improved photo-degradation efficiency could be attributed to the decomposition of PMS, which produced sulfate radicals and hydroxyl radicals due to the transmission of photo-generated electron/hole pairs’. The word ‘amount’ was deleted in the revised version.

• In line 29, please rewrite "...problem has attracted more and more attention..." instead of "problem has been paid more and more attention..."

The sentence was rewritten as ‘Owing to the rapid development of industry and fast growth of population, the increased water pollution problem has attracted much more attention.’.

• In line 31, type "brought" instead of "attracted"

In line 32, the word ‘attracted’ was changed to ‘brought’.

• In line 40, please rewrite the following sentence as: "leading to the formation of a series of reactive radicals".

In line 43, the sentence was rewritten as ‘On one hand, the heterogeneous catalysts were effective for the PMS activation of, leading to decomposition of PMS and formation of sulfate radicals.’.

• The sentence starting by: "In heterogeneous SR-photo process, PMS..." in line 43 is not correctly typed and is not well understood, please correct it.

In line 48, the sentence was rewritten as ‘In heterogeneous SR-photo-Fenton like process, PMS was not only an oxidant, but also an electron acceptor, which could prevent the recombination of photo-generated holes and electrons.’.

• In line 52, please retype it as: "...catalysts have received more attention..."

In line 57, the sentence was rewritten as ‘In recent years, Bi-based catalysts have received more attention which acted as the photocatalyst for the degradation of contaminants due to its unique properties and narrow band gap, facilitating in the application of visible light.’.

• In line 86, please rewrite the excerpt: "Then, both solutions were mixed..." In line 89, in the synthetic details, authors state that "CuBi2O4 was also prepared without glucose." Why do they do in this way? Some explanation would be welcome in this sentence.

In line 90, the excerpt was rewritten as following. The synthesis procedure of CuBi₂O₄ was no glucose addition in order to compare with the CuBi₂O₄-OVs catalyst under the same condition. The explanation was also typed in the revised manuscript.

‘A hydrothermal method was used to synthesize the CuBi₂O₄ catalyst with surface oxygen vacancy. 2 mM Bi(NO₃)₃·5H₂O and 1 mM Cu(NO₃)₂·3H₂O was dissolved in 10 mL of ethylene glycol, respectively. Then, both solutions were mixed under stirring. Afterward, 30 mL ethanol and 0.3 g of glucose was added to the mixed solution. After completely dissolved, the mixture was transferred into the 100 mL Teflon-lined stainless-steel autoclave and kept at 160 ^oC for 12 h. The solid was centrifuged and washed with deionized water and ethanol, which was dried at 70 ^oC for 24 h, which denoted as CuBi₂O₄-OVs. CuBi₂O₄ was also prepared without glucose. The glucose was used in the synthesis procedure which played a great role on the morphology of the nanocomposites due to the reduction, thus further improved the photocatalytic performance of the catalyst.’

• In line 111, TOC is mentioned for the first time without specifying its meaning.

We are sorry for the unclear writing. In line 119, the sentence was rewritten as ‘the concentrations of RhB and total organic carbon (TOC) were measured by UV-Vis spectrophotometer (UV 3600 II, Shanghai, China) and TOC (Elementar vario) analyzer, respectively’.

• In line 147, please type "introduction" instead of "introduce".

In line 160, the sentence was rewritten as ‘With the introduction of surface oxygen vacancy, the intensity of diffraction peaks was slightly decreased.’.

• Please, correct the double "the" in line 205.

In line 210, the double ‘the’ was deleted. The sentence was rewritten as ‘The prepared catalysts have no adsorption removal efficiency on RhB in the reaction time due to the limited BET surface area. And negligible removal of RhB was obtained by using PMS alone and using LED light alone, due to hardly production of reactive free radicals.’.

• In line 207: "The results might be attributed to..."

In line 220, the sentence was rewritten as ‘The results might attribute to the increased hydroxyl group on the surface, which made more reactive oxidant species in the heterogeneous SR-photo-Fenton like reaction.’

• In line 316, the word "hardly" would be better typed as "difficult"

In line 332, the word ‘hardly’ was replaced by the word ‘difficult’. The sentence was rewritten as ‘The EPR signals detected for DMPO-SO₄ was really weak due to its quick transition to hydroxyl radicals in the solution, which was difficult to capture the signal in the solution’.

 

We tried our best to improve the manuscript and made some changes in the manuscript. Your kind suggestions helped us to improve this manuscript a lot. We appreciate for reviewer’s warm advice earnestly, and hope that the correction will meet with approval.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper reported preparation of CuBi2O4 with oxygen vacancy by one-pot hydrothermal method, which is useful methodology to the used in the heterogeneous SR-photo reaction for the removal of RhB. Kinetics is well-written and characterization of the materials is well-performed.
Furthermore, mechanism of applications materials has been written rationally.

Therefore, I think this paper is suitable for the publication of Catalysts as its current form.

Author Response

Response to Reviewer 3 Comments

This paper reported preparation of CuBi2O4 with oxygen vacancy by one-pot hydrothermal method, which is useful methodology to the used in the heterogeneous SR-photo reaction for the removal of RhB. Kinetics is well-written and characterization of the materials is well-performed.

Furthermore, mechanism of applications materials has been written rationally.

Therefore, I think this paper is suitable for the publication of Catalysts as its current form.

Response: Thank you for your kind suggestion.

We have already carefully revised this manuscript in order to avoid the mistakes and wrong spelling problems. Thank you very much for your comments and suggestions. We tried our best to improve the manuscript and made some changes in the manuscript. Your kind suggestions helped us to improve this manuscript a lot. We appreciate for reviewer’s warm advice earnestly, and hope that the correction will meet with approval.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I really appreciate the efforts made by the authors to improve their manuscript.

I still have doubts about the usefulness of bismuth based photo-catalysts, but I really appreciate the comparison between the different catalysts (Table 1) in their response. I think that the table 1 and discussion should be added to the manuscript.

 

Author Response

Dear Reviewer:

Thank you for your comments concerning our manuscript entitled ‘Visible light driven spherical CuBi₂O₄ with surface oxygen vacancy enhanced photocatalytic activity: catalyst fabrication, performance and reaction mechanism’ (No.: catalysts-890665). Those comments are very helpful for revising our paper. We really appreciate your kind suggestions to help us improve the manuscript.

We have studied comments carefully and have made revisions which we hope meet with your approval. Revised texts are marked red in the revised manuscript.

Best regards.

Xin Zhong

E-mail: [email protected]

 

Please find the following response to the comments of referees.

Response to the reviewer’s comments. (Reviewer's comments marked in blue)

Reviewers' comments:

I really appreciate the efforts made by the authors to improve their manuscript.

I still have doubts about the usefulness of bismuth based photo-catalysts, but I really appreciate the comparison between the different catalysts (Table 1) in their response. I think that the table 1 and discussion should be added to the manuscript.

Response: Thank you for your kind suggestions and encourage.

We cherish your approval on our revised manuscript. For now, we could not perform the experiment by using xenon lamp as the light source. LED light was considered as an efficient alternative light source for removal of pollutants in wastewater due to its long working life, broad emission spectrum and cost-effective. We would like to explore the comparison experiments between using high-power xenon lamp and LED light as the light source in the further study to find out the application of Bi-based catalyst. We really appreciate your suggestions on our manuscript to help us improve the quality.

As reviewer’s suggestion, the discussion about the comparison between different Bi-based catalyst was added in the manuscript in line 66, Page 2, the table was added in the supplementary materials as Table S1.

‘Moreover, the photocatalytic performance by single usage of CuBi₂O₄ was unsatisfied, which suffered quick recombination of photogenerated holes and electrons. The degradation efficiency of the target pollutant was assumed to be further improved by using pure CuBi₂O₄ catalyst alone. On the other hand, to achieve better degradation efficiency, large amount of photocatalyst was utilized in these systems. It was reported that in the presence of PMS/PS which acted as the electron acceptor, the photocatalytic activity was much improved, while the comparison between the absence and presence of PMS/PS by the application of Bi-based catalyst was listed in Table S1.’.

We appreciate for reviewer’s warm advice earnestly. Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.pdf