Efficient Charge Transfer of p-n Heterojunction UiO-66-NH₂/CuFe₂O₄ Composite for Photocatalytic Hydrogen Production

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Photocatalytic hydrogen production is a current research focus. The manuscript entitled "Efficient Charge transfer of p-n heterojunction UiO-66-NH2/CuFe2O4 composite for photocatalytic hydrogen production" by Sekar et al described the synthesis and photocatalytic hydrogen production performance of a composite p-n heterojunction formed by UiO-66-NH2 and CuFe2O4.  The synthesized material was characterized by XRD, XANES, and SEM. The finding is interesting. While both UiO-66-NH2 and CuFe2O4 cannot produce hydrogen, the composite p-n heterojunction generate a substantial quantity of hydrogen generation due to the charge separation efficiency and charge carrier of 258 electrons and holes were enhanced. I suggest that this manuscript can be accepted for publication after some issues commented on below have been well addressed.

Major issues:
1. Hydrogen production efficiency is the most important concern. Reaction time should be considered. The hydrogen production performance should be described in μmol g-1 h-1 units instead of μmol g-1.
2. The solvent used to prepare the composite should be critical to forming heterojunctions, whether other solvents have been tried.
3. UiO-66-NH2 is a classic porous material, whose porosity needs to be characterized because its porosity often affects its performance.
4. Some works on UiO-66 performance modulation and porous composites photocatalytic performance should be properly cited, e.g., Molecules, 2022, 27, 8002; Chem. Eng. J., 2024, 479, 147605.

Minor issues:
1. The composite synthetic scheme is not numbered. The preparation of the composite material was not ground, but the mortar appeared in the scheme. It's confusing.
2. The abc lables are missing in Fig.6.

Author Response

Response to Reviewer -1

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

 

This article entitled “Efficient Charge transfer of p-n heterojunction 2 UiO-66-NH2/CuFe2O4 composite for photocatalytic hydrogen 3 production” prepared by Karthikeyan Sekar et al. describe the preparation of a novel UiO-66 (NH2)/CuFe2O4 p-n heterojunction material that has been applied as heterogeneous photocatalyst for hydrogen evolution. An extensive characterization of p-n heterojunction and pristine materials has been carried out by different instrumental techniques although the study of textural properties is missing. Furthermore, the reviewer considers that the fundamental weakness of this work lies in the study of photocatalytic activity, which is too scarce. However, the manuscript addresses a topic of potential interest to the scientific community and could be accepted in Catalysts journal after undergoing major revisions:

·         The abstract is poorly written with the ideas quite disorganized. Generally, this section should provide a general summary of the manuscript incorporating the relevant content obtained in the work in a logical order: from the statement of the problem, the procedure and characterizations carried out and, finally, the most relevant results obtained. This reviewer suggests that the authors redraft the abstract taking these indications into account.

·         Throughout the manuscript, capital letters in names such as "Gasification", "Thermochemical" or "Hydrogen", among others, should be replaced by lowercase letters where appropriate.

·         Lines 42-45: Most of the properties described for MOFs are suitable but others such as "enhanced visible absorption" cannot be included, since this is specifically acquired by some MOFs. If so, include examples providing references.

·         Lines 46-48: Provide references.

·         Lines 65-66: Provide references.

·         Lines 67-68: What is the bandgap of UiO-66? Include in this sentence.

·         From my perspective, the authors should report in the introduction section on the state of the art of p-n heterojunctions and their application in photocatalytic processes for hydrogen production, or photocatalysis in general. Thus, this work would be more contextualized.

·         Line 89: At the beginning of the results and discussion section, this reviewer suggests for better understanding that the authors refer to the prepared materials. That is, what are the starting materials and the p-n heterojunctions synthesized as catalysts, briefly indicating the synthesis methods.

·         Line 98: What is UN material? I understand that the pristine MOF Uio-66. Unify the nomenclature used.

·         Line 107: In CuO, Cu has oxidation state +2. The fit is related to XANES of Cu2O. Review this section and better discuss this technique. Why hasn't XANES been made from UCFO 50? Provide this information.

·         Fig 3.a: Better specify which material each figure corresponds to. It is described in a confusing way. What composite is it? UCFO 25 or 50? Provide both elemental mappings incorporating the atomic percentages.

·         Line 136-137: Provide references.

·         Line 149: Provide XPS of UiO-66 and UCFO 50.

·         Line 151-167: Explain in more detail how the calculation of the conduction band is carried out using the Nerst equation. How do the authors know the valence bands? It's all very confusing. Write again for better understanding of the reader. It is essential to elucidate the photocatalytic mechanism.

·         Line 176-192: Although the results are interesting, the photocatalytic study is too brief and poorly discussed. The reviewer proposes expanding this section by incorporating reaction kinetics and calculating the photocatalytic activity in terms of TON and TOF. Likewise, include a comparative table incorporating analogous p-n heterojunction materials applied in photocatalytic hydrogen evolution.

·         Line 193: Additionally, some post-reaction characterization of the UCFO p-n heterojunctions could also be carried out to confirm the photocatalytic stability of these materials.

·         Line 248: The experimental conditions of all electrochemical measurements should be included. This section must be properly detailed.

·         Reference the Supporting Information figures in the main manuscript. If they do not contribute anything relevant, it is better to eliminate them permanently.

 

Comments on the Quality of English Language

Many grammatical errors have been found throughout the manuscript. An extensive review of the English language is recommended.

Author Response

Response to Reviewer comment -2

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

This referee wishes to thank the authors for their response and added works. The authors have provided responses to the comments addressing several concerns. This revised manuscript fulfilled the most of requirements arisen from the first review. However, there are certain inconsistent points that should be reviewed again:

-          -In the introduction section, the state of the art of p-n heterojunctions and their application in photocatalytic processes for hydrogen production is somewhat poor. This should be somewhat more extended and better explained by providing more examples and some results.

-         -Although the synthesis scheme was presented in the materials and methods section, this reviewer suggested that the authors include an explanation of the materials synthesis process from the starting raw materials to the p-n heterojunctions. In this way, reading monitoring would be improved. This aspect has not been addressed by the authors.

-          -Several requested characterization data have not been provided (i.e. elemental mapping and XANES of UCFO 50). This reviewer considers it important when doing a comparative study between UCFO 25 and UCFO 50 materials.

-          How do the authors get the atomic percentages of UFCO 25? Provide this information in the manuscript.

-         - Some of the SI figures remain unreferenced in the manuscript. Please carry out this action and incorporate the corresponding text.

-          -The reviewer disagrees with the authors regarding the feasibility of TON and TOF calculations. TON refers to the moles of hydrogen produced per mole of catalyst. TOF is a parameter that means the TON per unit of time. Why is it not possible to calculate it? Even the best thing in this case is to provide a comparative table with respect to results obtained by other authors who used p-n hetejunctions based on MOF for hydrogen generation.

-          -This reviewer still thinks that although the results of the catalytic part are interesting, the photocatalytic study is too brief and poorly discussed.

-         - XRD diffractogram after reaction of the UFCO 25 material presents several additional diffraction planes compared to the starting material. Please identify these peaks and comment on them in the text.

These issues must be clarified before the publication of the manuscript in Catalysts.

Comments on the Quality of English Language

Some spelling errors were detected throughout the manuscript. A careful review is recommended to correct this issue.

Author Response

Attached 

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have provided a detailed and thorough response to the comments and have addressed the issues previously indicated. Therefore, this reviewer recommends this manuscript for publication in Catalysts journal