Mechanochemical Defect Engineering of Nb₂O₅: Influence of LiBH₄ and NaBH₄ Reduction on Structure and Photocatalysis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors, Anna Michaely, Elias C. J. Gießelmann, Guido Kickelbick, in the manuscript entitled “Mechanochemical Defect Engineering of Nb₂O₅: Influence of LiBH₄ and NaBH₄ Reduction on Structure and Photocatalysis” show that the aim of this research is to investigate the partial reduction of Nb₂O₅ through mechanochemical methods using lithium borohydride (LiBH₄) and sodium borohydride (NaBH₄) as reducing agents. This study seeks to enhance the electronic and photocatalytic properties of Nb₂O₅ by introducing controlled defects, such as oxygen vacancies and intercalated lithium ions, into its structure. The specific objectives include: Mechanochemical Reduction: To explore the effectiveness of LiBH₄ and NaBH₄ in reducing Nb₂O₅ under mechanochemical conditions.​ Structural and Electronic Characterization: To characterize the structural changes and electronic properties of the reduced Nb₂O₅ using techniques such as electron paramagnetic resonance (EPR) spectroscopy, powder X-ray diffraction (PXRD), Raman spectroscopy, and optical measurements.​ Photocatalytic Performance: To evaluate the photocatalytic activity of the reduced Nb₂O₅ samples in the degradation of methylene blue, comparing their performance to that of pristine Nb₂O₅.

 

The referee read the manuscript with interest and has some suggestions to improve merit of the manuscript.

To enhance and supplement your manuscript on Raman and UV-Vis absorbance spectroscopy of Nb₂O₅ during mechanochemical reduction, consider addressing the following questions. These can deepen your analysis, clarify mechanisms, and guide future experiments.

Please clarify and answer the following questions. I suggest that you insert the explanations into the work in the appropriate places.

Raman Spectroscopy:

1. How do changes in Raman peak intensity and width correlate with crystallite size and network strain during mechanochemical reduction?
2. Is the red shift of the LO mode specific to LiBH₄ reduction, or does it also occur with NaBH₄?
3. Can operando Raman spectroscopy track the dynamics of intercalation and deintercalation during cycling?
4. What role do oxygen vacancies play in the observed changes in Raman spectra?

 

UV-Vis Spectroscopy: Optical Property Modifications

1. How do oxygen vacancies and intercalation defects influence the optical band gap?
2. Is the observed band gap narrowing consistent across different synthesis methods?
3. How do defect-induced states contribute to photocatalytic activity?

 

I suggest that the authors, with their answers, expand the work and approach a wider number of readers and researchers.

I suggest that this paper be acceptable with minor technical changes.

Comments for author File: Comments.pdf

Author Response

Comment 1: I suggest that the authors, with their answers, expand the work and approach a wider number of readers and researchers.

Response 1: We thank the reviewer for these deeper questions. Please find below our answers to your questions:

 

Comment 2: How do changes in Raman peak intensity and width correlate with crystallite size and network strain during mechanochemical reduction?

Response 2: Typically, the peak width of Raman signals mainly increases with increasing structural disorder and deceasing crystallinity/crystallite size, while strain mainly leads to peak shifts. In literature, broadening and shifting of Raman signals are also often correlated with the presence of oxygen vacancies, which also induce structural disorder in the material. While it is not possible to identify the exact origin of the shifting and broadening of Raman signals, they can all be linked to structural changes induced during milling, which are often described in literature for partially reduced oxides. A short explanation has been added to the section “Raman and UV-Vis absorbance spectroscopy”.

 

Comment 3: Is the red shift of the LO mode specific to LiBH₄ reduction, or does it also occur with NaBH₄?

Response 3: The red shift is also observed for NaBH₄ reduction, but not as strongly. Our imprecise formulation has been revised.

 

Comment 4: Can operando Raman spectroscopy track the dynamics of intercalation and deintercalation during cycling?

Response 4: Yes, operando Raman spectroscopy can be used to investigate the (de)lithiation of Nb₂O₅ during cycling, as for example shown by Li et al. (https://doi.org/10.1021/acsenergylett.3c01031). Since we are not planning on doing any cycling experiments with our material and do not have the possibility to perform operando Raman spectroscopy, we chose not to include this method in our manuscript in order not to confuse the reader and to keep the current research focus.

 

Comment 5: What role do oxygen vacancies play in the observed changes in Raman spectra?

Response 5: See response to question 1.

 

Comment 6: How do oxygen vacancies and intercalation defects influence the optical band gap?

Response 6: Both oxygen vacancies and intercalation processes usually lead to a decrease of the optical band gap due to the formation of new electronic states within the band gap. A short explanation has been added to the section “Raman and UV-Vis absorbance spectroscopy”.

 

Comment 7: Is the observed band gap narrowing consistent across different synthesis methods?

Response 7: Yes, band gap narrowing is often reported for such partially reduced systems, independent of their synthesis method. Corresponding references have been added to the section “Raman and UV-Vis absorbance spectroscopy”.

 

Comment 8: How do defect-induced states contribute to photocatalytic activity?

Response 8: Due to the additional electronic states, absorption of visible light is facilitated, which leads to enhanced photocatalytic activity. Similarly, the presence of defects can prolong the lifetime of photogenerated charge carriers by hindering their recombination rate. Some effects of defects on photocatalytic activity have been added to the section “Photocatalytic degradation of methylene blue”.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper is focused on adjusting the properties of materials by incorporation defects into crystal structure. In this way the useful photocatalytic degradation properties can be enhanced. The authors chose the promising strategy based on mechanochemical method and reactive metal hydrides. The research was properly planned and carried out. Overall the text is carefully arranged and the paper structure is correctly organized. However, there are several points to improve the paper.

The mechanochemical methods usually introduce strains and defects during high energy milling. In this way the unit cell parameters and properties of pure Nb2O5 can be also slightly modified. The investigations of pure milled Nb2O5 materials should be included in the paper as the reference for performed investigations.

The authors have presented the results related to Nb2O5 however in the text they mentioned also TiO2 (e.g. line 60, line 157).

Author Response

Comment 1: The mechanochemical methods usually introduce strains and defects during high energy milling. In this way, the unit cell parameters and properties of pure Nb₂O₅ can be also slightly modified. The investigations of pure milled Nb₂O₅ materials should be included in the paper as the reference for performed investigations.

Response 1: We thank the reviewer for the valuable comments to improve the manuscript, and agree with him that high energy milling can induce strains and defects. Our milling conditions are rather soft, and we do not see significant changes after milling only Nb₂O₅ for 60 min. We chose 60 min of milling as reference, which was our longest tested milling time for the reduction, because we observed the greatest changes in microstructure at this milling time. We conclude from the reference experiment that the structural changes are mainly caused by the mechanochemical reduction. A discussion has been added on page 5.

 

Comment 2: The authors have presented the results related to Nb₂O₅ however in the text they mentioned also TiO₂ (e.g. line 60, line 157).

Response 2: We also mentioned TiO₂ since we investigated the reduction of both Nb₂O₅ and TiO₂ in our cited publication. We have removed the mention of TiO₂ to avoid confusion of the reader.