BiPO₄/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H₂O₂ Production

Round 1

Reviewer 1 Report

Abstract

1.      In title, please mention the word “Tetracycline”.

2.      Please provide the time taken to achieve maximum degradation for each catalyst used along with their rate constants.

Introduction

1.      Should specify why authors choose “tetracycline” specifically for photocatalytic studies?

2.      What are the other available methods previously employed? (In literature).

Materials and Methods

1.      Section 2.2 and 2.3. Preparation of pure and combination of catalyst was done based on the previously published literature or authors own method. Please specify, if it is based on previous literature, cite the proper references.

2.      Section 2.2, Why the BiOBr is labelled as Ov-BiOBr? I could not find any reason in the methodology.

Results and Discussion

1.      Figure 2, please incorporate the XRD spectra of pure BiPO₄, pure BiOBr, and all combinations of BiPO₄/BiOBr for better understanding.

2.      Section 3.4, Please mention few literature references, whether the same peaks are observed in the previously reported literature.

3.      Section 3.6. Authors have prepared different percentage combinations of BiOBr with BiPO₄, however most characterization studies speaks about only one particular combination (10%), should also highlight how different ratios might affect their properties.

4.      Section 3.7. Authors should extend the number of cycles, as even after 3 cycles the 90% efficiency is achieved. Can extend the study until it goes below 50%.

5.      Section 3.7. Authors have stated that the degradation follows “first-order reaction kinetics”, therefore please mention the rate constant for all the samples used (not only 10% BiPO₄/BiOBr).

6.      Authors should compare the obtained results with those already reported for similar studies (tetracycline visible light degradation).

Conclusion

1.      Authors have stated “BiPO₄ content had a significant impact on the catalyst performance, with the optimal addition amount being 10 mol%”. However section 2.3. says BiPO₄ concentration is constant and BiOBr concentration varies (10 - 40%). So based on this, it is the BiOBr concentration which is being making the impact rather than the BiPO₄, as the concentration is constant in the combined catalyst preparation. Rectify the sentence accordingly.

References

1.      Should include more references, as only 20 out of 53 were used in results and discussion section. That to most of them were used sparsely.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1 Comments

Abstract:

1. In title, please mention the word “Tetracycline”.

Response: Thank you for your good suggestion. “Tetracycline” has been added to the title.

2. Please provide the time taken to achieve maximum degradation for each catalyst used along with their rate constants.

Response: Thank you for your reminder. The time required to achieve maximum degradation for each catalyst and their rate constants have been added to Abstract.

Introduction：

1. Should specify why authors choose “tetracycline” specifically for photocatalytic studies?

Response: Thank you for your suggestion. The reasons for choosing 'tetracycline' for photocatalytic research have been added to the section of Introduction.

2. What are the other available methods previously employed? (In literature).

Response: Thank you for your question. Various methods have been employed for tetracycline degradation, such as: Biodegradation, Photochemical degradation, Advanced oxidation processes (AOPs), Mem-brane filtration technology and Adsorption. However, these methods have drawbacks such as slow degradation rates, high energy consumption, susceptibility to environ-mental conditions, generation of secondary pollutants, stringent operating conditions, only achieving separation and concentration rather than true degradation of pollutants, reaching adsorption saturation easily, and the potential for membrane fouling.

Materials and Methods:

1. Section 2.2 and 2.3. Preparation of pure and combination of catalyst was done based on the previously published literature or authors own method. Please specify, if it is based on previous literature, cite the proper references.

Response: Thank you for your suggestion. The preparation of pure Ov-BiOBr in Section 3.2 is based on previous literature, specifically references [13][28][34], which have been added to Section 3.2. The preparation of composites in Section 3.3 is based on the hydrothermal method for other materials, with various parameters adjusted to achieve the best preparation results. (In accordance with the journal's requirements, the experimental and conclusion sections have been adjusted, and the previously mentioned Sections 2.2 and 2.3 have been changed to Sections 3.2 and 3.3).

2. Section 2.2, Why the BiOBr is labelled as Ov-BiOBr? I could not find any reason in the methodology.

Response: Thank you for your question. The reason why BiOBr is labeled as Ov-BiOBr is based on the previous literature [13][28][34]. By using a reductive solvent, ethylene glycol, in the synthesis process of the pure catalyst from the precursor, elemental Bi is reduced from [Bi₂O₂]²⁺, thus creating oxygen vacancies in the lattice.

Results and Discussion:

1. Figure 2, please incorporate the XRD spectra of pure BiPO₄, pure BiOBr, and all combinations of BiPO₄/BiOBr for better understanding.

Response: Thank you for your suggestion. Pure BiPO₄, pure BiOBr, and all BiPO₄/BiOBr combinations have been added to the XRD spectrum in Figure 1. According to journal requirements, the experimental and conclusion sections have been reversed, and the previously mentioned Figure 2 has been changed to Figure 1.

Figure. 1 XRD patterns of Ov-BiOBr, BiPO₄ and BiPO₄/Ov-BiOBr composites

2. Section 3.4, Please mention few literature references, whether the same peaks are observed in the previously reported literature.

Response: Thank you for your question. In the referenced literature in Section 2.4 [42][43], three peaks at 56 cm^-1, 112 cm^-1, and 374 cm^-1 were also observed. Among them, the band at 56 cm^-1 can be attributed to the first-order scattering A_1g mode of Bi. (In accordance with the journal's requirements, the experimental and conclusion sections have been adjusted, and the previously mentioned Section 3.4 has been changed to Section 2.4)

3. Section 3.6. Authors have prepared different percentage combinations of BiOBr with BiPO₄, however most characterization studies speaks about only one particular combination (10%), should also highlight how different ratios might affect their properties.

Response: Thank you for your suggestion. The effect of different proportions on the characterization performance of the catalyst has been added in Section 2.6. (In accordance with the journal's requirements, the experimental and conclusion sections have been adjusted, and the previously mentioned Section 3.6 has been changed to Section 2.6).

4. Section 3.7. Authors should extend the number of cycles, as even after 3 cycles the 90% efficiency is achieved. Can extend the study until it goes below 50%.

Response: Thank you for your suggestion. Based on your recommendation, the cycling experiment was redone. After 39 cycles and 3.25 days, the degradation efficiency of the catalyst decreased to 45% (Figure. 8). However, it still maintained a certain level of catalytic efficiency, indicating the catalyst is relatively stable.

Figure. 8 Photodegradation curve (a) and photodegradation kinetic curve (b) of TC; photodegradation cycle test curve (c) and XRD detection before and after illumination of BiPO4/Ov-BiOBr (d)

5. Section 3.7. Authors have stated that the degradation follows “first-order reaction kinetics”, therefore please mention the rate constant for all the samples used (not only 10% BiPO₄/BiOBr).

Response: Thank you for your suggestion. The rate constants (k) for all samples have been listed in Section 2.7. (In accordance with the journal's requirements, the experimental and conclusion sections have been adjusted, and the previously mentioned Section 3.7 has been changed to Section 2.7)

6. Authors should compare the obtained results with those already reported for similar studies (tetracycline visible light degradation).

Response: Thank you for your good suggestion. The comparation of the results of this study with those of similar reported studies (visible light degradation of tetracycline) is presented in Table 1.

Table 1. Recent work of BiOBr-based heterojunctions in degradation of TC

Conclusion:

1. Authors have stated “BiPO₄ content had a significant impact on the catalyst performance, with the optimal addition amount being 10 mol%”. However section 2.3. says BiPO₄ concentration is constant and BiOBr concentration varies (10 - 40%). So based on this, it is the BiOBr concentration which is being making the impact rather than the BiPO₄, as the concentration is constant in the combined catalyst preparation. Rectify the sentence accordingly.

Response: Thank you very much for pointing out the errors in the description. This paper discusses the impact of the addition of BiPO₄ on the performance of the composite, and the preparation process of the composite has been re-narrated in Section 3.3. (In accordance with the journal's requirements, the experimental and conclusion sections have been adjusted, and the previously mentioned Section 2.3 has been changed to Section 3.3)

References:

1. Should include more references, as only 20 out of 53 were used in results and discussion section. That to most of them were used sparsely.”

Response: Thank you for your suggestions and reminders. More references have been cited in the section of Results and discussion.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript, the authors reported that BiPO4/Ov-BiOBr high-low junction for efficient visible light photocatalytic degradation and H2O2 production performance. The authors claimed that the composite catalyst with a molar content of 10% BiPO4 exhibited the highest degradation rate of tetracycline (TC), which was 2.3 times that of Ov-BiOBr. 10%BiPO4/Ov-BiOBr displayed the best photocatalytic activity for producing H2O2, which was 3.6 times that of Ov-BiOBr and 19 times that of BiPO4. This was due to the appropriate bandgap matching between BiPO4 and Ov-BiOBr, the photo-generated electron transfer channel via Bi-bridge, and efficient charge separation. In overall, this manuscript is interesting but in order to consider publication, this work should be revised. The following comments should be addressed for the improvement of their manuscript.

Comment 1: The overall study aims for this study about the formation of Bi-PO4/Ov-BiOBr heterojunction composite material through a two-step solvothermal method for efficient visible light photocatalytic degradation and H2O2 production performance need to be further clarified in detail.

Comment 2: The various recent reports and their research findings on the “Bi-PO4/Ov-BiOBr heterojunction composite material” using various of promising methods and their visible light photocatalytic degradation performance should be summarized into a table form and discussed for better understanding in term of benchmarking points with your research findings.

Comment 3: The role and impact of lattice defects, oxygen vacancies and element partitioning within the Bi-PO4/Ov-BiOBr heterojunction composite material in improving the visible light photocatalytic degradation and H2O2 production performance need to be further clarified with fundamental support. 

Comment 4: The carefully English correction is necessary for the whole manuscript. Please check and revise accordingly.

Author Response

Response to Reviewer 2 Comments

In this manuscript, the authors reported that BiPO₄/Ov-BiOBr high-low junction for efficient visible light photocatalytic degradation and H₂O₂ production performance. The authors claimed that the composite catalyst with a molar content of 10% BiPO₄ exhibited the highest degradation rate of tetracycline (TC), which was 2.3 times that of Ov-BiOBr. 10%BiPO₄/Ov-BiOBr displayed the best photocatalytic activity for producing H₂O₂, which was 3.6 times that of Ov-BiOBr and 19 times that of BiPO₄. This was due to the appropriate bandgap matching between BiPO₄ and Ov-BiOBr, the photo-generated electron transfer channel via Bi-bridge, and efficient charge separation. In overall, this manuscript is interesting but in order to consider publication, this work should be revised. The following comments should be addressed for the improvement of their manuscript.

1. The overall study aims for this study about the formation of Bi-PO₄/Ov-BiOBr heterojunction composite material through a two-step solvothermal method for efficient visible light photocatalytic degradation and H₂O₂ production performance need to be further clarified in detail.

Response: Thank you for your feedback and suggestions. The overall research objectives related to the formation of BiPO₄/Ov-BiOBr heterojunction composites using a two-step solvothermal method to improve the visible-light photocatalytic performance for TC degradation and H₂O₂ generation have been elaborated in more detail in the section of Introduction.

2. The various recent reports and their research findings on the “Bi-PO₄/Ov-BiOBr heterojunction composite material” using various of promising methods and their visible light photocatalytic degradation performance should be summarized into a table form and discussed for better understanding in term of benchmarking points with your research findings.

Response: Thank you for your suggestion. The comparison of the results of this study with those of similar reported studies (visible light degradation of tetracycline) is presented in Table 1.

Table 1. Recent work of BiOBr-based heterojunctions in degradation of TC

3. The role and impact of lattice defects, oxygen vacancies and element partitioning within the Bi-PO₄/Ov-BiOBr heterojunction composite material in improving the visible light photocatalytic degradation and H₂O₂ production performance need to be further clarified with fundamental support.

Response: Thank you for your comments and suggestions. In this study, oxygen vacancies were introduced by using a reducing agent, ethylene glycol, during the synthesis of the composites. The Bi²⁺ ions in the [Bi₂O₂]²⁺ layer were reduced to metallic Bi, forming a Bi bridge between BiPO₄ and Ov-BiOBr, which acted as a transfer channel for photogenerated electrons.[26][34][48] This facilitated the reaction between the photogenerated electrons and the substances adsorbed on the surface of the catalyst, leading to the production of free radicals and the degradation of pollutants, as well as the generation of H₂O₂. The existence of metallic Bi was confirmed by XRD, Raman spectroscopy, and XPS. The PO₄^3- in BiPO₄ played a role in inducing the formation of space charge regions between the two catalysts, which resulted in a bending of the energy band structure and facilitated the transfer of photogenerated electrons under illumination.[17][43]

[17] Liu, Z.S.; Wu, B.T.; Niu, J.N.; Feng, P.Z.; Zhu, Y.B. BiPO₄/BiOBr p–n Junction Photocatalysts: One-Pot Synthesis and Dramatic Visible Light Photocatalytic Activity. Materials Research Bulletin 2015, 63, 187–193, doi:10.1016/j.materresbull.2014.12.020.

[26] Sun, Z.; Yang, X.; Yu, X.-F.; Xia, L.; Peng, Y.; Li, Z.; Zhang, Y.; Cheng, J.; Zhang, K.; Yu, J. Surface Oxygen Vacancies of Pd/Bi₂MoO_6-x Acts as “Electron Bridge” to Promote Photocatalytic Selective Oxidation of Alcohol. Applied Catalysis B: Environmental 2021, 285, 119790, doi:10.1016/j.apcatb.2020.119790

[34] Li, X.; Liu, Q.; Deng, F.; Huang, J.; Han, L.; He, C.; Chen, Z.; Luo, Y.; Zhu, Y. Double-Defect-Induced Polarization En-hanced O_V-BiOBr/Cu_2−xS High-Low Junction for Boosted Photoelectrochemical Hydrogen Evolution. Applied Catalysis B: Environmental 2022, 314, 121502, doi:10.1016/j.apcatb.2022.121502.

[43] Zou, X.; Dong, Y.; Zhang, X.; Cui, Y.; Ou, X.; Qi, X. The Highly Enhanced Visible Light Photocatalytic Degradation of Gaseous o-Dichlorobenzene through Fabricating like-Flowers BiPO₄/BiOBr p-n Heterojunction Composites. Applied Surface Science 2017, 391, 525–534, doi:10.1016/j.apsusc.2016.06.003.

[48] Deng, F.; Zhang, Q.; Yang, L.; Luo, X.; Wang, A.; Luo, S.; Dionysiou, D.D. Visible-Light-Responsive Graphene-Functionalized Bi-Bridge Z-Scheme Black BiOCl/Bi₂O₃ Heterojunction with Oxygen Vacancy and Multiple Charge Transfer Channels for Efficient Photocatalytic Degradation of 2-Nitrophenol and Industrial Wastewater Treatment. Applied Catalysis B: Environmental 2018, 238, 61–69, doi:10.1016/j.apcatb.2018.05.004.

4. The carefully English correction is necessary for the whole manuscript. Please check and revise accordingly.

Response: Thank you for your suggestion. Comprehensive revisions and corrections have been made.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper deals with the development of BiPO4/Ov-BiOBr for tetracycline degradation under visible light. There are numerous characterizations, the figures are really well presented, and globally the work is very well-executed. The paper needs minor revisions before publication, here are some minor comments:

-          In the experimental section, the concentration of TC is not mentioned. Moreover, the blank and dark tests must be also mentioned to ensure that no direct photolysis of TC is observed and that adsorption has been taken into account.

-          A comparison with other photocatalysts about the TC degradation is missing in the results & discussion part.

Author Response

Response to Reviewer 3 Comments

This paper deals with the development of BiPO₄/Ov-BiOBr for tetracycline degradation under visible light. There are numerous characterizations, the figures are really well presented, and globally the work is very well-executed. The paper needs minor revisions before publication, here are some minor comments:

1. In the experimental section, the concentration of TC is not mentioned. Moreover, the blank and dark tests must be also mentioned to ensure that no direct photolysis of TC is observed and that adsorption has been taken into account.

Response: Thank you for your comments and suggestions. In the experimental section, the concentration of TC has been added as 50 mg·L^-1. The missing descriptions for the blank and dark experiments have been newly added to the experimental section. According to the results of photocatalytic performance for TC degradation, it can be observed that TC did not undergo direct photolysis, and the adsorption has been taken into consideration.

2. A comparison with other photocatalysts about the TC degradation is missing in the results & discussion part.

Response: Thank you for your suggestion. The comparison of the results of this study with those of other photocatalysts about the TC degradation is presented in Table 1.

Table 1. Recent work of BiOBr-based heterojunctions in degradation of TC

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

In overall, this manuscript was technically well revised. This revised manuscript meets the criteria of Catalysts. Therefore, in my opinion, the revised manuscript can be accepted for publication.