Visible Light-Driven Photocatalytic Degradation of Tetracycline Using p-n Heterostructured Cr₂O₃/ZrO₂ Nanocomposite

Round 1

Reviewer 1 Report

Check the attachment.

Comments for author File: Comments.pdf

Minor editing of English language required.

Author Response

Reviewer #1: Xueyu Wei et al. prepared a Cr₂O₃/ZrO₂ photocatalyst using co-precipitation method. The Cr₂O₃/ZrO₂ composite was applied toward the degradations of toxic tetracycline (TCL) under visible light. However, some of the claims of this study are not substantiated by the experimental results. The authors did not use enough citations to explain their work. Furthermore, the quality of the present form of this paper is not commensurate with the high standards of the journal Water. Therefore, the manuscript needs a major revision. The following aspects need improvement, and the comments are expected to be discussed in the text before the publication.

 

Q1. The authors should carefully check the typo and space errors throughout the manuscript.

Reply: Based on the reviewer’s comment, we have carefully checked the possibility grammatical errors that were present in the manuscript, with appropriate grammatical and tenses changes carried out in the revised manuscript.

Q2. The authors prepared Cr₂O₃ nano-cubes by the calcined process at a high temperature (900°C). Normally, synthesizing any product at high temperature is not an easy and cost-effective method. So, why the authors are used to synthesize at high temperatures?

Reply: The aim of the study is to prepare metal oxide hetero junction which is mostly involved high temperatures and not possibly get the pure and crystalline form of some metal oxides with green methods. High-temperature synthesis is a broad category of processes used to create materials and compounds under elevated temperature conditions. These processes are employed in various scientific, industrial, and materials science applications to achieve specific properties, structures, and compositions. Here are some common high-temperature synthesis methods:

Solid-State Synthesis:

In this method, reactants in solid form, often powders, are heated to high temperatures to facilitate chemical reactions. This process is used to produce a wide range of materials, including ceramics, intermetallic compounds, and refractory materials.

Sol-Gel Process:

The sol-gel method involves the synthesis of materials from a solution (sol) that undergoes gelation to form a gel. This gel is then heated to produce materials such as glasses, ceramics, and thin films. The process allows for precise control of composition and structure.

Hydrothermal Synthesis:

In hydrothermal synthesis, materials are synthesized in a high-temperature and high-pressure aqueous environment. This method is often used to create crystalline materials, including metal oxides, zeolites, and other complex compounds with unique properties.

CVD (Chemical Vapor Deposition):

CVD involves the chemical reaction of vaporized precursors at high temperatures to form thin films, coatings, or bulk materials. It is widely used in the semiconductor and materials industries.

Laser Ablation:

High-energy lasers are used to ablate a target material, producing vapor or plume, which can be deposited on a substrate to create thin films or nanoparticles.

Melt Synthesis:

Materials are formed by melting precursor compounds and allowing them to cool and solidify. This method is used for producing metallic alloys, glasses, and various other materials.

Flame Synthesis:

Combustion processes in flames are used to synthesize materials like nanoparticles and functional coatings. Examples include flame spray pyrolysis and flame hydrolysis.

Combustion Synthesis:

Combustion processes can be harnessed to synthesize materials, including ceramics and nanomaterials. Self-propagating high-temperature synthesis (SHS) is an example.

Arc Discharge and Spark Plasma Sintering:

These methods utilize electrical discharges and high-temperature, high-pressure conditions to produce materials with unique properties.

High-Temperature Reduction:

Some compounds, such as metal oxides, can be reduced to the pure metal form at high temperatures. This process is commonly used in metallurgy.

The specific method chosen depends on the desired material and its properties, as well as the required temperature and pressure conditions. Careful control of parameters such as temperature, time, and precursor chemistry are essential to obtain the desired material with the correct properties. High-temperature synthesis is used in a wide range of applications, from semiconductor manufacturing to advanced ceramics and nanomaterial production.

References

[a] M.G. Tsegay, H.G. Gebretinsae, Z.Y. Nuru, Structural and optical properties of green synthesized Cr2O3 nanoparticles, Materials Today: Proceedings, 36 (2021) 587-590

[b] Zhenzhao Pei, Hongbin Xu, Yi Zhang, Preparation of Cr2O3 nanoparticles via C2H5OH hydrothermal reduction,

Journal of Alloys and Compounds, 468 (2009) L5-L8

[c] Yangang Wang, Xiaohong Yuan, Xiaohui Liu, Jiawen Ren, Weiyi Tong, Yanqin Wang, Guanzhong Lu, Mesoporous single-crystal Cr2O3: Synthesis, characterization, and its activity in toluene removal, Solid State Sciences

10 (2008) 1117-1123

Q3. Instrumentation should be written in detail.

Reply:  The detailed instrumentation has been mentioned in the revised manuscript (Section 2.3 of the revised manuscript)

Q4. For the reusability experiment, describe how the catalyst samples were collected after each trial.

Reply: As per the suggestion, the catalyst recycle test description has been added in the revised manuscript. (Section 3.3 of the revised manuscript)

Q5. The preparation process of ZrO₂ be included in section 2. Because they used ZrO₂ for all the experiments

Reply: The preparation of ZrO₂ is as follows. Initially, Zirconium (IV) tetra-butoxide (≥ 99.99%, Sigma Aldrich) precursor was mixed with 15 mL solution of H₂O₂ (30% w/v), followed by stirring for 0.5 hours, resulting in a white-colored colloidal peroxo complex of Zirconium. To the above suspension, 50 mL of distilled water was added and the suspension was stirred for 24 hours. Further, the complete suspension was then transferred to a Teflon lined autoclave maintained at 100°C for 12 hours. After bringing the autoclave to room temperature the catalyst was washed several times with water, dried at 80 °C followed by calcination at 500 °C for 2 h by increasing the temperature at 5 °C/min. The final powder was denoted as Cr₂O₃/ZrO₂ nanocomposite. (Section 2.2 of the revised manuscript)

Q6. Which plane is used to measure the crystalline size? The authors should clarify it. Lines 53-54, without proper explanation, these were mere statements. So, the authors must explain the reason behind these statements (why and how this value is correlated with this manuscript?).

Reply: We are confused with the reviewer’s question as the Line 53-54 is about the well-known statement about Advanced Oxidation Process. We assumed that the reviewer was asking about Line 153-154. The crystalline size of the prepared composite has been carried out selecting the major intense peaks and their average size has been calculated using Scherrer formula. As per the suggestion this sentence has been included in the revised manuscript. (Section 3.1 of the revised manuscript)

Q7. The diffuse reflectance UV-Vis-spectroscopy data must be included in the manuscript.

Reply: As per the suggestion the UV-vis data has been included in the revised manuscript (Inlet of Fig. 2a of the revised manuscript)

 

 

Q8. Lines 192-194, very confusing statement. The authors said “The band gap energy of the nanocomposite (Cr₂O₃/ZrO₂) is calculated using the Kublkamunk technique and extrapolated from Tauc's plot” but Fig. 2a looks Tauc's plot. In addition, the Fig. 2a caption is not correct.

Reply: As per the suggestion, the sentence has been modified in the revised manuscript.

Q9. Lines 199-208, need references.

Reply: As per the suggestion the references have been included in the revised manuscript. (References 29-31 of the revised manuscript).

[a] M.N. Yazdi, Y. Yamini, H. Asiabi, Multiwall carbon nanotube- zirconium oxide nanocomposite hollow fiber solid phase microextraction for determination of polyaromatic hydrocarbons in water, coffee and tea samples, J. Chromatogr. A. 1554 (2018) 8–15. https://doi.org/10.1016/j.chroma.2018.04.040.

[b] T. Ivanova, K. Gesheva, A. Cziraki, A. Szekeres, E. Vlaikova, Structural transformations and their relation to the optoelectronic properties of chromium oxide thin films, J. Phys. Conf. Ser. 113 (2008) 1–6. https://doi.org/10.1088/1742-6596/113/1/012030.

[c] M. Ocaa, Nanosized Cr2O3 hydrate spherical particles prepared by the urea method, J. Eur. Ceram. Soc. 21 (2001) 931–939. https://doi.org/10.1016/S0955-2219(00)00288-0.

 

Q10. Lines 214-215, the authors said “The results clearly evidenced that there is a formation of Cr₂O₃/ZrO₂ heterojunction during coprecipitation process (Figure 3a-b).” From Fig. 3a-b, how the authors claimed it is a formation of Cr₂O₃/ZrO₂ heterojunction because it is not Cr₂O₃/ZrO₂.

Reply: Sorry for the mistake, we have corrected the figuring place in the revised manuscript. (Section 3.1 of the revised manuscript)

Q11. How do they measure the average size of Cr₂O₃ nanoparticles? Also, how many particles were counted? It is better to add a particle-size histogram.

Reply: Based on the TEM analysis results, the average particle size of Cr₂O₃, ZrO₂, and Cr₂O₃/ZrO₂ NCs were calculated using Image J software. The corresponding values are 36.5 nm, 27.5 nm, and 30.5 nm, respectively. We thank the reviewer for this suggestion, which helped us to provide the information. As per the suggestion the results have been included in the revised manuscript

Q12. Lines 2018-220, must need references.

Reply: As per the suggestion, the references have been included in the revised manuscript. (Reference 10, 32 of the revised manuscript)

[a]         N.K. Sompalli, A. Mohanty, A.M. Mohan, P. Deivasigamani, Heterojunction Cr2O3-Ag2O nanocomposite decorated porous polymer monoliths a new class of visible light fast responsive heterogeneous photocatalysts for pollutant clean-up, J. Environ. Chem. Eng. 9 (2021) 104846. https://doi.org/10.1016/j.jece.2020.104846 .

[b]        D. Jagadeesan, N.K. Sompalli, A.M. Mohan, C.V.S.B. Rao, S. Nagarajan, P. Deivasigamani, ZrO2–Ag2O nanocomposites encrusted porous polymer monoliths as high-performance visible light photocatalysts for the fast degradation of pharmaceutical pollutants, Photochem. Photobiol. Sci. 21 (2022) 1273–1286. https://doi.org/10.1007/s43630-022-00218-y .

Q13. The Fig. 3 caption is not correct. Check it carefully.

Reply: Sorry for the error, as per the suggestion the figure caption has been corrected in the revised manuscript.

Q14. In the XPS explanation, must need references.

Reply: As per the suggestion, the references have been included in the revised manuscript. (Reference 33-36 of the revised manuscript)

[a]        A. Kadari, T. Schemme, D. Kadri, J. Wollschläger, XPS and morphological properties of Cr₂O₃ thin films grown by thermal evaporation method, Results Phys. 7 (2017) 3124–3129. https://doi.org/10.1016/j.rinp.2017.08.036 .

[b]        Z. Cao, C. Zuo, Cr₂O₃/carbon nanosheet composite with enhanced performance for lithium ion batteries, RSC Adv. 7 (2017) 40243–40248. https://doi.org/10.1039/c7ra06188a .

[c]        J. Liu, M. Liao, M. Imura, A. Tanaka, H. Iwai, Y. Koide, Low on-resistance diamond field effect transistor with high-k ZrO₂ as dielectric, Sci. Rep. 4 (2014) 2–6. https://doi.org/10.1038/srep06395 .

[d]        J. Luo, X. Luo, C. Hu, J.C. Crittenden, J. Qu, Zirconia (ZrO₂) Embedded in Carbon Nanowires via Electrospinning for Efficient Arsenic Removal from Water Combined with DFT Studies, ACS Appl. Mater. Interfaces. 8 (2016) 18912–18921. https://doi.org/10.1021/acsami.6b06046 .

Q15. I recommended adding the point of zero charge of the Cr₂O₃/ZrO₂ catalyst.

 

Reply: As per the suggestion the point zero charge has been included in the revised manuscript. (Reference 3.2 of the revised manuscript)

To demonstrate the surface charge of the Cr₂O₃/ZrO₂ nanocomposite, the zeta potential at various pH is measured to elucidate the isoelectric point of the photocatalyst, as shown in Fig.S1, with an isoelectric point at pH 6.22. The enhanced photocatalytic degradation of TCL at pH 5.0 is attributed to the strong electrostatic interaction of the positively charged surface of the Cr₂O₃/ZrO₂ photocatalyst, with the anionic tetracycline drug molecule. Beyond the isoelectric point (> 6.22) the decline in photocatalytic drug degradation efficiency is attributed to the electrostatic repulsive between the negatively charged photocatalyst surface and the anionic tetracycline drug molecules.

 

Q16. Kinetics study is also an important result in photocatalytic activity evaluation. I suggest adding these data to the manuscript.

Reply: The kinetics study has been included in our original manuscript in which different parameters have been studied on the degradation efficiency of the as prepared catalysts in degrading TCL antibiotic under visible light and the optimal conditions were achieved to further analyze the plausible degradation products generated during the treatment process (Fig. 5 and Fig. 6 of the original manuscript)

 

Q17. The authors must compare their photocatalytic activity with the published work. Without comparing how do the readers know your product is better?

Reply: The revised manuscript includes a comparison table between the present work and the existing literature based on the reviewer's suggestions. We thank the reviewer for this suggestion, which helped us to provide the information.

Table. 1 Comparison of literature reports on the degradation of Tetracycline drug with the proposed method

 

S. No.	Photocatalyst	Light Source	Degradation Kinetics	Catalyst Amount (mg)	Drug Conc. (mg/L)	Ref.
1.	α-Fe2O3 /g-C3N4 NCs	Visible (32 mW/cm2, W)	180 min	50	10	[32]
2.	Black Phosphorus/BiOBr NCs	Visible (300 W/cm2, Xe)	90 min	100	50	[33]
3.	BiWO6	Visible (300 W/cm2, Xe)	180 min	30	20	[34]
4.	WO3/g-C3N4	Visible (300 W/cm2, Xe)	60 min	50	80	[35]
5.	(Bi)BiOBr/rGO NCs	Visible (300 W/cm2, Xe)	140 min	50	20	[36]
6.	Ni-WO3	LED (35W)	105 min	50	10	[37]
7.	BiVO4/Fe2O3	Visible (300 W/cm2, Xe)	60 min	30	15	[38]
8.	CuS/CdS NCs	Solar Light Source	50 min	50	20	[39]
9.	Cr2O3/ZrO2	Visible (300 W/cm2, Xe)	120 min	100	50	Present Work

 

Q18. The stability and reusability of the compound are very important for the practical application of a catalyst. The authors did the reusability. Therefore, it would be better if they added the structural changes of the catalyst after the photocatalytic reaction.

Reply: As per the suggestion the structural changes of the photocatalyst before and after recycles has been investigated through XRD analysis. And the XRD results reveals an unaltered crystallinity along with retention of the structural characteristics of the photocatalyst even after five cycles of usage. (Fig. 9b of the revised manuscript)

 

Q19. It is highly recommended to add the BET analysis (surface area and pore size) for all the samples.

Reply: As per the suggestion the BET and pore size analysis has been included in the revised manuscript. The BET and pore size distribution analysis were conducted on the prepared samples to determine the surface area and pore characteristics. The surface area of bare Cr₂O₃ is 23.83 m²/g with pore volume and pore diameter of 0.033 cm³/g and 4.05nm, respectively. Similarly, the BET surface area of pristine ZrO₂ nanoparticles is 14.74 m²/g, including a pore volume and diameter of 0.0012 cm³/g and 2.90 nm. However, the Cr₂O₃/ZrO₂ nanocomposite exhibits an increased surface area of 38.43m²/g as well as a mid-way pore volume of 0.025 cm³/g and pore diameter of 4.35 nm. The increased surface area and pore properties of the Cr₂O₃/ZrO₂ nanocomposite confirm that ZrO₂ nanoparticles have been dispersed onto the surface of Cr₂O₃ nanoparticles rather than intercalated into the lattice. (Section 3.1 of the revised manuscript)

 

Q20. In section 2, the authors must add the details procedure of the LC-ESI/MS experiment.

Reply: As per the suggestion the details has been included in the revised manuscript. (Section 2.4 of the revised manuscript)

Q21. There are so many errors in formatting the references. All references should be presented according to the style of the journal format.

Reply: The manuscript has been thoroughly checked for any grammatical and formatting errors and corrected in the revised manuscript. 

Author Response File: Author Response.docx

Reviewer 2 Report

 

1.      What is the commercial significance of this present work? Scientific explanations should be made on this subject.

2.      The authors have conducted the photocatalytic activity of the dyes at a lab scale only. Why did they not employ real-time samples for the study?

3.      The abstract and conclusion sections should be updated to enhance the quality of the manuscript.

4.      Typographical errors are present throughout the manuscript. Authors are required to pay keen attention to this.

5.      a.u. is not an appropriate abbreviation for arb. units as a.u. stands for astronomical units. Author should be change it.

6.      The catalyst should be compared to some of the well-established catalysts, and this is solely lacking.

            7.      A wide variety of drug are available worldwide. Why did the author choose tetracycline drug in particular?  

 

 

 

·         Check for grammatical errors and refine the draft.

 

·         A minor revision to the language is essential. Kindly recheck and modify the tenses.

Author Response

Reviewer #2:

‎Q1.  What is the commercial significance of this present work? Scientific explanations should be made on this subject.

Reply: In terms of wastewater treatment for environmental protection, semiconductor photocatalysis is undoubtedly a useful tool. However, preparing nano-semiconductor materials with high quality has some extensive challenges. To improve the efficiency of photocarrier transfer and prevent electron and hole recombination, it is necessary to seek new methods of modification. In the field of photocatalysis, visible-light-responsive photocatalysts are preferable because visible light is the main source of solar energy. Thus, heterojunction construction and surface modification have been widely employed to enhance electron generation, separation, and utilization. However, despite Cr₂O₃'s advantageous properties, very few reports have been published regarding its use as a direct photocatalyst. Furthermore, the strategic selection of Cr₂O₃ is based on a waste-to-resources conversion strategy, in which carcinogenic hexavalent chromium compounds that are utilized in various industrial applications can be converted into benign trivalent chromium oxide that can be used as a photocatalyst for environmental remediation. Thus, Cr₂O₃ has been envisioned as an alternative photoactive material through a modified method of coupling it with ZrO₂, a stable metal oxide.

“Based on the reviewer's suggestion, the above statement has been refined and included in the revised manuscript.”

References

[a] N.K. Sompalli, A. Mohanty, A.M. Mohan, P. Deivasigamani, Heterojunction Cr2O3-Ag2O nanocomposite decorated porous polymer monoliths a new class of visible light fast responsive heterogeneous photocatalysts for pollutant clean-up, J. Environ. Chem. Eng. 9 (2021) 104846.

 

[b] J.Z.Y. Tan, F. Xia, M.M. Maroto-Valer, Raspberry-Like Microspheres of Core–Shell Cr2O3@TiO2 Nanoparticles for CO2 Photoreduction, ChemSusChem. 12 (2019) 5246–5252.

 

[c] J. Singh, V. Verma, R. Kumar, Preparation and structural, optical studies of Al substituted chromium oxide (Cr2O3) nanoparticles, Vacuum. 159 (2019) 282–286.

 

Q2.   The authors have conducted the photocatalytic activity of the dyes at a lab scale only. Why did they not employ real-time samples for the study?

Reply: The present work focuses on the preparation of highly efficient photocatalyst materials for degradation of antibiotic pollutant Tetracycline (TCL) and we have not used any dyes in our study. We agree with the reviewer's suggestions, However, we are still in the early stages of our research, and these are our preliminary results and we would like to be considering this to extend the study to real-time samples of urban water environments. Specifically, we are focusing on the rivers and sewage treatment plants during different rainfall events and comparing the differences during dry weather.  The results of this type of detailed study will demonstrate the presence tetracycline pollutant in real water samples. Following the detailed study analysis of real-time samples, we have concluded this work on laboratory samples and will be focusing on real-time water samples in the future. Lot many works have also been demonstrated in the similar manner

References

[a] Stella, R.J.; Sreevani, I.; Gurugubelli, T.R.; Ravikumar, R.V.S.S.N.; Koutavarapu, R. Enhanced Solar Light-Driven Photocatalytic Degradation of Tetracycline Using Fe3+-Doped CdO/ZnS Nanocomposite: Mechanistic Insights and Performance Evaluation. Catalysts 2023, 13, 1312. https://doi.org/10.3390/catal13091312

 

[b] Wang, N.; Zhao, Y.; Wu, X.; Li, D.; Ma, R.; Chen, Z.; Wu, Z. Synthesis of Cu Nanoparticles Incorporated Mesoporous C/SiO2 for Efficient Tetracycline Degradation. Nanomaterials 2023, 13, 2478. https://doi.org/10.3390/nano13172478

 

[c] C. Cao, Z. Lu, L. Wang, L. Li, D. Deng, D. Liao, C. Mo, Ce-doped BiFeO3 as a photocatalyst to enhance photo-Fenton degradation of tetracycline, Journal of Photochemistry and Photobiology A: Chemistry 446 (2024) 115161

‎Q3.  The abstract and conclusion sections should be updated to enhance the quality of the manuscript.

Reply: The abstract and conclusion sections of the manuscript have been revised based on the reviewer's suggestions to improve the manuscript's quality

‎Q4.  Typographical errors are present throughout the manuscript. Authors are required to pay keen attention to this.

Reply: We thank the reviewer for pointing out the typographical mistakes, which have been rectified in the revised manuscript.

‎Q5.  a.u. is not an appropriate abbreviation for arb. units as a.u. stands for astronomical units. Author should be change it.

Reply: The unit of absorbance has been modified in accordance with the reviewer's suggestion, and we thank the reviewer for pointing out this error.

 ‎
‎Q6.  The catalyst should be compared to some of the well-established catalysts, and this is solely lacking.

Reply: The revised manuscript includes a comparison table between the present work and the existing literature based on the reviewer's suggestions. We thank the reviewer for this suggestion, which helped us to provide the information.

 

Table. 1 Comparison of literature reports on the degradation of Tetracycline drug with the proposed method

 

S. No.	Photocatalyst	Light Source	Degradation Kinetics	Catalyst Amount (mg)	Drug Conc. (mg/L)	Ref.
1.	α-Fe2O3 /g-C3N4 NCs	Visible (32 mW/cm2, W)	180 min	50	10	[32]
2.	Black Phosphorus/BiOBr NCs	Visible (300 W/cm2, Xe)	90 min	100	50	[33]
3.	BiWO6	Visible (300 W/cm2, Xe)	180 min	30	20	[34]
4.	WO3/g-C3N4	Visible (300 W/cm2, Xe)	60 min	50	80	[35]
5.	(Bi)BiOBr/rGO NCs	Visible (300 W/cm2, Xe)	140 min	50	20	[36]
6.	Ni-WO3	LED (35W)	105 min	50	10	[37]
7.	BiVO4/Fe2O3	Visible (300 W/cm2, Xe)	60 min	30	15	[38]
8.	CuS/CdS NCs	Solar Light Source	50 min	50	20	[39]
9.	Cr2O3/ZrO2	Visible (300 W/cm2, Xe)	120 min	100	50	Present Work

 

Q7. A wide variety of drug are available worldwide. Why did the author choose tetracycline drug in particular? 

Reply: Tetracycline is one of the most widely used broad-spectrum antibiotics for the treatment of animals and poultry. As these medicines cannot be fully digested by organisms, their release into the environment through animal residues and metabolites is often observed in industrialized foods, such as eggs, milk, and meat, which are released through urine and feces, which contaminate soils and water bodies. It has been reported that TCL concentrations are high in industrial effluents, surface waters, effluents from ETP units, effluents from sewage treatment plants, and other environmental compartments. Moreover, since Tetracycline is chemically stable, it is difficult to eliminate under natural conditions. Additionally, the use of TCL antibiotics can cause dangerous health problems such as hepatotoxicity, allergic reactions, chronic poisoning, and gastroenteritis. Therefore, the eradication of these substances from the aqueous environment is essential and urgent. As a result of all the above statements, tetracycline has been selected as a modern organic pollutant for the present study.

References

[a]          R. Daghrir, P. Drogui, Tetracycline antibiotics in the environment: A review, Environ. Chem. Lett. 11 (2013) 209–227.

[b]          X. Peng, W. Luo, J. Wu, F. Hu, Y. Hu, L. Xu, G. Xu, Y. Jian, H. Dai, Carbon quantum dots decorated heteroatom co-doped core-shell Fe0@POCN for degradation of tetracycline via multiply synergistic mechanisms, Chemosphere. 268 (2021).

[c]          X. Zheng, Y. Liu, X. Liu, Q. Li, Y. Zheng, A novel PVDF-TiO2@g-C3N4 composite electrospun fiber for efficient photocatalytic degradation of tetracycline under visible light irradiation, Ecotoxicol. Environ. Saf. 210 (2021).

Q8. Comments on the Quality of English Language Check for grammatical errors and refine the draft. Also, minor revision to the language is essential. Kindly recheck and modify the tenses.

Reply: Based on the reviewer’s comment, we have carefully checked the possibility grammatical errors that were present in the manuscript, with appropriate grammatical and tenses changes carried out in the revised manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

This paper show tetracycline was removed by visible light-driven photocatalytic degradation through p-n heterostructured Cr₂O₃/ZrO₂ nanocomposite. The mechanisms about the processes of removing tetracycline by p-n heterostructured Cr₂O₃/ZrO₂ nanocomposite processes were investigated by several characterization techniques. The studies were quite systematic and the resulted were well organized by the authors. I’d like to recommend the publication of this paper in water after revision.

1.       In UV-Vis-DRS spectra, the author should explain why Cr₂O₃ catalyst are a wave curve for checking the catalyst impurity.

2.       In FTIR spectra, the author should explain why nature hydroxyl groups (at 3450 cm^-1) of Cr₂O₃/ZrO₂ nanocomposite is the lowest among all samples for confirm whether the oxygen vacancy is in the structure.

3.       Based on HR-TEM image, the average particles size of Cr₂O₃ catalyst and ZrO₂ catalyst should be provided for identifying them.

4.       According to XPS results, the author should provide the element ratios for understanding the relationship between of Cr₂O₃ catalysts and ZrO₂ catalysts.

Author Response

Reviewer #3: This paper show tetracycline was removed by visible light-driven photocatalytic degradation through p-n heterostructured Cr₂O₃/ZrO₂ nanocomposite. The mechanisms about the processes of removing tetracycline by p-n heterostructured Cr₂O₃/ZrO₂ nanocomposite processes were investigated by several characterization techniques. The studies were quite systematic and the resulted were well organized by the authors. I’d like to recommend the publication of this paper in water after revision.

Q1. In UV-Vis-DRS spectra, the author should explain why Cr₂O₃ catalyst are a wave curve for checking the catalyst impurity.

Reply: Due to its octahedral geometrical preferences, the absorption spectra for pure Cr₂O₃ NPs exhibit three major peaks. This is the reason for the curved wave pattern on Tauc's plot of the corresponding band gap energy of the Cr₂O₃ NPs. The primary peak is associated with the Cr₂O₃ (d³ system) of octahedral symmetry comprising two different energy levels namely, an occupied triply degenerate (T_2g) state and an unoccupied doubly degenerate (E_g) state that accounts for the d-d electronic transitions. The other two spectral peaks for Cr₂O₃ NPs are attributed to intrinsic electronic transitions from ⁴A_2g →⁴T_1g and ⁴A_2g →⁴T_2g of Cr³⁺ ions. Moreover, XRD results confirmed the purity of Cr₂O₃ nanoparticles.

References

[a]          C.P. Ireland, R.G. Palgrave, S.C. Bennett, A.W.J. Smith, J.H. Clark, J.R. Darwent, J.B. Claridge, S. Poulston, M.J. Rosseinsky, Visible light photocatalysis by metal-to-metal charge transfer for degradation of methyl orange, J. Mater. Chem. A. 4 (2016) 12479–12486.

Q2. In FTIR spectra, the author should explain why nature hydroxyl groups (at 3450 cm^-1) of Cr₂O₃/ZrO₂ nanocomposite is the lowest among all samples for confirm whether the oxygen vacancy is in the structure.

Reply: As a result of the formation of heterostructures, atmospheric CO₂ will react with the surface hydroxyl groups of Cr₂O₃/ZrO₂, which will result in the consumption of hydroxyl groups at vibrational band at 3450 cm^-1. We thank the reviewer for this point, which helped us to provide the information.

References

[a] S. Song, J. Wei, X. He, G. Yan, M. Jiao, W. Zeng, F. Dai, M. Shi, Oxygen vacancies generated by Sn-doped ZrO2 promoting the synthesis of dimethyl carbonate from methanol and CO2, RSC Adv. 11 (2021) 35361–35374.

 

Q3. Based on HR-TEM image, the average particles size of Cr₂O₃ catalyst and ZrO₂ catalyst should be provided for identifying them.

Reply: Based on the TEM analysis results, the average particle size of Cr₂O₃, ZrO₂, and Cr₂O₃/ZrO₂ NCs were calculated using Image J software. The corresponding values are 36.5 nm, 27.5 nm, and 30.5 nm, respectively. We thank the reviewer for this suggestion, which helped us to provide the information.

Q4.  According to XPS results, the author should provide the element ratios for understanding the relationship between of Cr₂O₃ catalysts and ZrO₂ catalysts.

Reply: In response to the reviewer's concern, we have provided atomic percentages of the elements present in the nanocomposite based on the XPS analysis results. According to the XPS results, the atomic percentages of the nanocomposite elements are found to be Chromium (Cr³⁺) - 25.5%, Zirconium (Zr⁴⁺) - 12.3%, Oxygen (O) - 41.7% and Carbon (C) – 20.5%, confirming the uniform distribution of the elements and the formation of a heterostructure between them.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors revised the manuscript very well. The manuscript has significantly improved in its quality. 

But, a few corrections are needed before accepting. 

1. The authors must include the BET data (Figure).

2. In Table 1, they must be added the degradation percentage.

3. Also, the Table 1 caption is not correct. Degradation kinetics should be replaced with degradation time.

English language fine. 

Author Response

Q1. The authors must include the BET data (Figure).

Reply: Added BET figure accordingly (Figure 5 of the revised manuscript)

Q2. In Table 1, they must be added the degradation percentage.

Reply: Added the degradation percentage in the revised Table 1

Q3. Also, the Table 1 caption is not correct. Degradation kinetics should be replaced with degradation time.

Reply: Corrected accordingly

Reviewer 2 Report

In revised form, this manuscript justified all comments and has improved significantly to justify a publication

Author Response

Q1. In revised form, this manuscript justified all comments and has improved significantly to justify a publication

Reply: Many thanks for your suggestions to improve the quality of our manuscript