Efficiency of Microwave-Assisted Surface Grafting of Ni and Zn Clusters on TiO₂ as Cocatalysts for Solar Light Degradation of Cyanotoxins

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript titled "Ni and Zn clusters on TiO2 as cocatalysts for solar light degradation of cyanotoxins" presents an innovative approach to modifying TiO2 with metal clusters for photocatalytic degradation, and the data provided are substantial. However, there is a need for more comprehensive characterization and discussion of some key factors in the photocatalytic degradation process. Here are my suggestions:

1. If the Ni and Zn clusters, as mentioned in the introduction, are several nanometers in size, they should be referred to as nanoparticles. Additionally, it is essential to confirm the composition and structure of the catalyst before discussing its catalytic activity and analyzing the mechanisms.

2. There is insufficient evidence for the existence, size, and location of Ni and Zn clusters.

3. From the perspective of catalytic mechanism research, SiO2 should be removed through etching, as its presence is detrimental to revealing the mechanism of catalytic degradation.

4. The degradation process is an oxidation process, and Ni and Zn metals are easily oxidized. Do Ni and Zn clusters as co-catalysts directly participate in the degradation process of cyanotoxins? If they are involved in the oxidation process, how is the stability of Ni and Zn clusters ensured? If they do not participate in the reaction, what is the role of Ni and Zn clusters?

5. The electron transfer direction between Ni and Zn clusters and TiO2 needs to be discussed, as it is crucial for elucidating the photocatalytic degradation mechanism of TiO2/Ni or Zn.

Author Response

Comment 1: If the Ni and Zn clusters, as mentioned in the introduction, are several nanometers in size, they should be referred to as nanoparticles. Additionally, it is essential to confirm the composition and structure of the catalyst before discussing its catalytic activity and analyzing the mechanisms.

Answer 1: We believe the reviewer is referring to the lines 84–86 of the original manuscript stating, “microwaves are also used in synthesis to improve the size distribution of the grafted nanoparticles (NPs) or clusters and to enhance structural and morphological properties for the nanomaterials.” We did insinuate the clusters to be of nanometer size in the last part of abstract in the sentence “grafting few nm large metal species to TiO₂”. We changed this sentence to “grafting sub-nano-sized metal species to TiO₂.” We have recently published a paper with similar clusters, albeit on ZnO, and the size was also proven to be around one nm (https://doi.org/10.1016/j.apsusc.2024.161463).

In this paper we are refraining from the more traditional form of characterization first and catalytic results latter. The composition and synthesis of materials are introduced in the last paragraph of the introduction just enough so that the reader knows what the discussion is about and then the underlying reasons for the observed behaviour of the catalysts is discussed as in the chronological order the research is being conducted. That is, the most interesting materials are then subject to in-depth analysis to explain the structure–activity relationship.

Comment 2: There is insufficient evidence for the existence, size, and location of Ni and Zn clusters.

Answer 2: The evidence on existence as well as location is given with elemental mapping done on TEM imaging of the samples. We have, however, included the previously missing XPS data for Zn 2p ½ peak in sample TiO₂-Zn 0.5%. This is now shown in ESI file Fig. S6, and the Zn 2p1/2 peak shows the presence of what is most likely TiO₂-Zn. The strongest Zn peak, the 2p3/2 peak at 1022 could not be used as it partially overlaps with the final O KLL Auger peak. Unfortunately, when it comes to Ni-modified samples little to no Ni is seen in XPS data. The detection limit of the XPS is around 1000 ppm. The Ni 3s peak was used because the other main Ni peaks overlapped with other prominent peaks which can be seen in the survey scan (fluorine). The 2p peaks (the strongest and typically used) overlap with the F Auger peaks. The 3p peak overlaps with a Na peak. Nevertheless, the location of both Ni and Zn oxocluters is seen in TEM imaging and clusters are evenly distributed on the surface of TiO₂ particles.

Comment 3: From the perspective of catalytic mechanism research, SiO₂ should be removed through etching, as its presence is detrimental to revealing the mechanism of catalytic degradation.

Answer 3: This is an interesting comment. We quite like the idea of it from the mechanistic point of view, indeed. However, apart from the characterization involving powders (XRD, N₂ sorption, etc.) all other measurements were conducted on films to measure properties of the applied system as much as possible. In this case, etching would most probably dissolve also the clusters, especially Zn, as it is quite pH unstable. SiO₂ is also inert and does not interfere in the electron transport nor does it compete with the incoming photons.

Comment 4: The degradation process is an oxidation process, and Ni and Zn metals are easily oxidized. Do Ni and Zn clusters as co-catalysts directly participate in the degradation process of cyanotoxins? If they are involved in the oxidation process, how is the stability of Ni and Zn clusters ensured? If they do not participate in the reaction, what is the role of Ni and Zn clusters?

Answer 4: Indeed, they are easily oxidized, that is why their nature is metal-oxo-clusters. In this form they are a stable co-catalyst on the surface of TiO₂. They are not participating in oxidation part of the photocatalytic process but rather in reduction part. This is shown in the quenching experiments where the largest effect on kinetics had p-benzoquinone (O₂^•– scavenger), showing the instrumental role of dissolved O₂ on the degradation of MC-LR. This was corroborated with additional EPR measurements (Fig. 5C). In this manner, the presence of metal-oxo-clusters on titania surface stabilized photogenerated e– and showed better resilience (less of an effect) to the presence of superoxide radical scavenger. Their stability lies in their oxide nature and has been proven with up to 4 consecutive runs (Fig. 6A).

Comment 5: The electron transfer direction between Ni and Zn clusters and TiO₂ needs to be discussed, as it is crucial for elucidating the photocatalytic degradation mechanism of TiO₂/Ni or Zn.

Answer 5: The e^– transfer is now more thoroughly discussed in lines 254–267.

Reviewer 2 Report

Comments and Suggestions for Authors

Review Report on the Manuscript " catalysts-3428998-peer-review-v1: Ni and Zn clusters on TiO₂ as cocatalysts for solar light degradation of cyanotoxins”. The submitted manuscript explores the synthesis and application of Ni and Zn bimetallic clusters on TiO₂ for photocatalytic degradation of cyanotoxins under visible light. While the study provides valuable insights into microwave-assisted grafting and catalyst optimization, it is essential to highlight some limitations and areas for improvement according to the following points:

1. The results indicate that bimetallic NiZn clusters on TiO2 do not show any synergistic enhancement in photocatalytic performance compared to single-metal systems. Instead, bimetallic clusters exhibit lower photocurrent and increased recombination rate. The reason of this conclusion should be clear in the manuscript

2. The kinetic constant for the degradation of MC-LR (microcystin-LR) with bimetallic catalysts is lower than that of single-metal catalysts. This raises concerns about the practical utility of bimetallic modifications when simpler single-metal systems perform better.

3. Line 290: A delay of approximately 10 minutes in the onset of cyanotoxin degradation is observed in modified TiO2 samples. While the manuscript attributes this delay to competition for adsorption sites and radical mechanisms, this phenomenon might hinder the efficiency of rapid water treatment systems where time-sensitive processes are crucial.

4. The study notes that Zn modification at low concentrations exhibits adverse effects on photocatalytic performance. These negative impacts are unexplained and require further investigation to understand their origin.

5. The photocatalytic activity is highly sensitive to the loading of Ni and Zn clusters. Deviations from the optimum values significantly reduce activity, limiting the robustness of the system under varying experimental or operational conditions.

6. The absence of Ti³⁺ or oxygen vacancies, which often contribute to synergistic effects in TiO2-based photocatalysis. Additionally, the exact mechanism behind the increased recombination rate in bimetallic systems wasn’t explained.

7. Although extensive characterization techniques such as SEM, TEM, and XPS are employed, the study lacks quantitative analysis of the metal clusters due to limitations in EDXS, XPS, and EELS methods.

8. EIS analysis should be completed by equivalent circuit analysis (Suggested reference on how you could do this analysis: Journal of Alloys and Compounds 816, 152513).

While the manuscript contributes to the understanding of metal-modified TiO2 photocatalysts, the study is limited by the lack of synergy in bimetallic systems, unexplained mechanistic behaviors, and performance constraints. Addressing these limitations through advanced characterization and mechanistic studies could enhance the manuscript.

Author Response

Comment 1: The results indicate that bimetallic NiZn clusters on TiO₂ do not show any synergistic enhancement in photocatalytic performance compared to single-metal systems. Instead, bimetallic clusters exhibit lower photocurrent and increased recombination rate. The reason of this conclusion should be clear in the manuscript.

Answer 1: The remark from the reviewer is valid, since it turns out the message wasn’t communicated clearly enough. The fact that the combination of the two metals produced negative results was an additional but necessary step in the whole study. Several new sentences were added to reclaim this statement, namely in lines 280–284 and in the first sentence of the abstract.

Comment 2: The kinetic constant for the degradation of MC-LR (microcystin-LR) with bimetallic catalysts is lower than that of single-metal catalysts. This raises concerns about the practical utility of bimetallic modifications when simpler single-metal systems perform better.

Answer 2: The comment is valid. It was not clearly stated that the intention was to study all possible combinations of clusters. Hence the two most promising metals from our previous research (https://doi.org/10.1039/C7TA07176K) were tested in different reaction (MC-LR), and additionally the combination of bimetal clusters was also synthesised for the first time. Negative results arose, but we still think it is necessary to report those also – a thing seen to seldom in scientific reports. Last sentence of the introduction was modified to state the intentions more clearly.

Comment 3: Line 290: A delay of approximately 10 minutes in the onset of cyanotoxin degradation is observed in modified TiO₂ samples. While the manuscript attributes this delay to competition for adsorption sites and radical mechanisms, this phenomenon might hinder the efficiency of rapid water treatment systems where time-sensitive processes are crucial.

Answer 3: Indeed, this is very likely to happen. We mentioned this concern now in lines 335–337. Although we cannot at this point test the rapid aquatic system, the next best thing was to test the materials in real water matrix, which were exactly the tests following Chapter 2.3.

Comment 4: The study notes that Zn modification at low concentrations exhibits adverse effects on photocatalytic performance. These negative impacts are unexplained and require further investigation to understand their origin.

Answer 4: Excellent observation. However, apart from some speculative mechanism, there isn’t much we can offer at this point. Such investigation would probably require a new set of tests and detailed characterization at such low levels of grafted species. Nevertheless, this notion was emphasised in line 190 in the revised version of the manuscript.

Comment 5: The photocatalytic activity is highly sensitive to the loading of Ni and Zn clusters. Deviations from the optimum values significantly reduce activity, limiting the robustness of the system under varying experimental or operational conditions.

Answer 5: True. This can also be considered a benefit when scaling up the synthesis. However, this notion was included in the conclusions in lines 498–500.

Comment 6: The absence of Ti³⁺ or oxygen vacancies, which often contribute to synergistic effects in TiO₂-based photocatalysis. Additionally, the exact mechanism behind the increased recombination rate in bimetallic systems wasn’t explained.

Answer 6: Indeed. The presence of V_O would probably greatly improve the absorption of VIS light and as such also the generation of charge carriers, as explained in https://www.sciencedirect.com/science/article/pii/S0169433216324102?via%3Dihub. This was already mentioned in the original manuscript in lines 215–221. The mechanism regarding the observed increased recombination rate of NiZn-TiO2 catalyst was now further investigated and described in lines 280–240. These results were also corroborated with additional EPR study showing no detectable generation in the bi-metal modified sample under VIS illumination, lines 304–316.

Comment 7: Although extensive characterization techniques such as SEM, TEM, and XPS are employed, the study lacks quantitative analysis of the metal clusters due to limitations in EDXS, XPS, and EELS methods.

Answer 7: The reviewer made a valid point. The quantitative analysis was not done. It would require ICP elemental analysis to check real values compared to starting stoichiometric ratios. Yet, our previous research on similar mono-metal modified samples showed good agreement with the nominal values (https://doi.org/10.1039/C7TA07176K). The presence of both metals was proven with HR-TEM, while we strongly believe the more important parameter here is the size and distribution of clusters on the surface which was shown to be even in both metal modifications.

Comment 8: EIS analysis should be completed by equivalent circuit analysis (Suggested reference on how you could do this analysis: Journal of Alloys and Compounds 816, 152513).

Answer 8: We rewrote the text regarding the EIS analysis following the advice by the reviewer. The text now goes as follows:

The electrochemical impedance spectra shown in Fig. 4A were analysed by fitting them to an electrochemical equivalent circuit (EEC). This model consists of the solution resistance (R_S), the charge transfer resistance (R_CT), the Warburg impedance (W) and a constant phase element (CPE) (Fig. S7) [https://doi.org/10.1016/j.apsusc.2021.152196]. A lower R_CT value indicates a more efficient charge carrier transfer process [https://doi.org/10.1016/S1872-2067(20)63783-4], resulting in a longer "lifetime" of the charge carriers. As can be seen in Table X, pure TiO₂ exhibited a high R_CT value of 1.263 MΩ. However, the incorporation of Ni and Zn significantly reduced the R_CT values, with the TiO₂-Zn sample having the lowest R_CT value of 0.258 MΩ among the materials analysed.

Reviewer 3 Report

Comments and Suggestions for Authors

The article of Andraž Šuligoj, Mallikarjuna Nadagouda, Gregor Žerjav,  Dionysios D. Dionysiou, Nataša Novak Tušar is devoted to a relevant and important topic - degradation of cyanotoxins under sunlight illumination using modified Ni and Zn TiO2. However, there are a number of comments that need to be corrected before the article is published.

1. In the abstract and conclusions, authors report on the synthesis of Ni–Zn bimetallic clusters on the surface of TiO2. However, the authors further point out that their photocatalytic activity is significantly lower than that of TiO2 modified with one metal. The abstract and conclusions should indicate the novelty and originality of the study, since modified titanium dioxide has been used as a photocatalyst for many years.

2. The sentence in the abstract “Bi-metal modified titania showed lower photocurrent compared to bare TiO2 and increased the recombination rate of such samples under the visible light irradiation, probably due to creation of tight junctions that could serve as recombination centers.” is unclear. What does it mean -  “recombination rate of such samples”? How can samples recombine? How can tight junctions be recombination centers? This is not clear. The introduction and conclusions should be written clearly.

3. Lines 250-251. “The samples used in this test were modified with 1 wt.% for better comparison...”. It's not clear 1 wt.% of what? It should be written clearly. The caption to the figure 4 also doesn't make this clear. 

4. In paragraph 2.4 “Recyclability & river water test” the authors indicate figures 7a-d, this is a mistake, there is no such figure. There is figure 6.

5. The authors did not receive direct evidence of the presence of O2•– radicals. I would recommend using a direct method for diagnosing oxygen anion radicals - electron paramagnetic resonance spectroscopy.

6. Have you done a degradation profile of the reaction with real river water with zinc modified TiO2?

 

Author Response

Comment 1: In the abstract and conclusions, authors report on the synthesis of Ni–Zn bimetallic clusters on the surface of TiO₂. However, the authors further point out that their photocatalytic activity is significantly lower than that of TiO₂ modified with one metal. The abstract and conclusions should indicate the novelty and originality of the study, since modified titanium dioxide has been used as a photocatalyst for many years.

Answer 1: It is indeed true that in these samples, single metal modification was shown superior to bimetal one. However, one would immediately ask for the bimetal modification if it was not made, hence it is imperative to include it in the results. For this reason, the abstract was modified in several positions as indicated with the red font in the revised manuscript.

Comment 2: The sentence in the abstract “Bi-metal modified titania showed lower photocurrent compared to bare TiO₂ and increased the recombination rate of such samples under the visible light irradiation, probably due to creation of tight junctions that could serve as recombination centers.” is unclear. What does it mean -  “recombination rate of such samples”? How can samples recombine? How can tight junctions be recombination centers? This is not clear. The introduction and conclusions should be written clearly.

Comment 2: True, some lines in the abstract were unclear. Which made the whole conclusions of the manuscript less convincing. We corrected them accordingly, especially the one mentioned in this question. We corrected “bare TiO₂” to “single metal-grafted TiO₂” since the previous version was not correct. Also, from the reviewer’s question it is obvious that the message was not clearly delivered. Hence, we corrected the sentence regarding recombination rate. The rate was referring to rate of e^–/h⁺ recombination and not sample’s recombination.

Bi-metallic clusters may act as recombination centers by if the two metals do not form a favourable electron transfer pathway, i.e., they can create shallow trap states rather than effective charge separation sites. For example, if one metal has a weak affinity for trapping electrons, it may retain charge carriers long enough for recombination instead of transferring them to reactants. This is not likely in our case due to the following. We postulated in our previous paper (https://doi.org/10.1039/C7TA07176K) that in terms of electron recombination ability such grafted species act partially as their corresponding oxides. Hence, Ea of NiO is 1.47 eV (or even 3.05 eV for NiO₂) while for ZnO it is 2.08 eV (CRC Handbook of Chemistry and Physics, 2005, David R. Lide, ed., CRC Press, Boca Raton, FL, 2005). This is rather close but different which should facilitate the transfer of e^–. A more plausible explanation is excessive cluster formation where large bi-metallic clusters may reduce active surface area and act as recombination sites where both photogenerated electrons and holes accumulate. This happens when metal particles aggregate instead of being finely dispersed on TiO₂. Figure 2 M and N shows distinctively larger formations of both Ni and Zn than when single metal modification was made (Fig. 2 D and I).

Comment 3: Lines 250-251. “The samples used in this test were modified with 1 wt.% for better comparison...”. It's not clear 1 wt.% of what? It should be written clearly. The caption to the figure 4 also doesn't make this clear. 

Answer 3: The sentence was rewritten to “The samples used in this test were loaded with 1 wt.% of metal clusters (Ni, Zn, and NiZn) for better comparison and since the bimetallic modification showed a negative contribution to the MC-LR degradation kinetics at this concentration.” Also in the Figure 4, the mention of metal clusters was added.

Comment 4: In paragraph 2.4 “Recyclability & river water test” the authors indicate figures 7a-d, this is a mistake, there is no such figure. There is figure 6.

Answer 4: Indeed. Sharp eyes of the reviewer saw a mistake where the reference should be to Figure 6 and not 7. The mistake was corrected in line 347.

Comment 5: The authors did not receive direct evidence of the presence of O₂^•– radicals. I would recommend using a direct method for diagnosing oxygen anion radicals - electron paramagnetic resonance spectroscopy.

Answer 5: The comment was taken into account and EPR measurements were conducted on powder samples. They are included in Fig. 5C and a second paragraph in Section “2.3 Study of active species” was added with the discussion on the proof of O₂^•– formation under VIS illumination. As shown, Ni-modified ZnO showed prominent signal for superoxide, which is consistent with its redox position close to O₂/superoxide, as discussed in the text.

Comment 6: Have you done a degradation profile of the reaction with real river water with zinc modified TiO₂?

Answer 6: Unfortunately, the degradation was done only with the Ni-modified sample as indicated in the text. This was done due to its high stability (Fig. 6A) and overall fastest kinetics. As it turned out, this sample was also the only one with measurable superoxide generation (previous answer) under visible light irradiation.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It can be accepted now.

Author Response

The reviewer 1 was happy fine the first revision already.

Reviewer 2 Report

Comments and Suggestions for Authors

I have reviewed the revised manuscript titled "Ni and Zn Clusters on TiO₂ as Cocatalysts for Solar Light Degradation of Cyanotoxins." After evaluating the initial submission and the authors responses to my previous comments, I regret to inform you that the manuscript is still not suitable for publication. My decision is based on the following scientific concerns:

1. The study clearly demonstrates that bimetallic NiZn clusters on TiO₂ do not enhance photocatalytic performance compared to single-metal systems. The authors attempted to clarify this issue in the revision, but the response ultimately indicate a negative impact rather than an improvement.
2. The kinetic constant degradation of MC-LR with bimetallic catalysts is lower than that of single-metal catalysts. So, the practical significance of the bimetallic modification is not possible.
3. The observed 10-minute delay in cyanotoxin degradation due to competitive adsorption and radical formation mechanisms is a drawback. The authors ail to provide any solution.
4. The study notes that Zn modification at low concentrations leads to adverse photocatalytic performance, but the exact mechanism remains unexplained. The authors claim that further investigation is required but do not provide any new data or insights in the revision.
5. The study does not explore whether Ti³⁺ species or oxygen vacancies contribute to photocatalytic activity.
6. Despite employing SEM, TEM, and XPS, the authors did not perform a quantitative analysis of the metal clusters. The authors refer to previous work, this does not replace the need for direct quantitative validation in the current study.
7. The authors have revised their EIS analysis following my suggestion in the first review, but the interpretation of charge transfer dynamics remains insufficiently detailed. The revised section lacks a comprehensive discussion of the equivalent circuit elements in photocatalytic performance.
8. The authors added a new co-author to the revised manuscript. This raises questions regarding research ethics and proper attribution of contributions. Author lists should be finalized before submission.

Given these unresolved comments and ethical issues, I recommend the rejection decision.

Comments on the Quality of English Language

It should be revised.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf