A Facile Synthesis of Bi₂O₃/CoFe₂O₄ Nanocomposite with Improved Synergistic Photocatalytic Potential for Dye Degradation

Round 1

Reviewer 1 Report

The article focuses on a fundamental problem of environmental protection and materials that can reduce the pollution of ecosystems. I agree with the authors that water pollution is a major environmental problem. That is why research on new materials for the removal of dye pollutants is so important. The proposal to use a Bi₂O₃/CoFe₂O₄ nanocomposite for this purpose is very interesting and promising. The preparation of the described material and the selection of the experiments do not raise any doubts, but I have reservations about the presentation of research results and preparation of the publication.

My comments are listed below:

1. Figure 2 presents SEM images of (a) Bi2O3; (b) CoFe2O4; (c) NC-1; (d) NC-2; (e) NC-3, but the resolution of scanning electron micrographs c, d, and e is very low. The authors wrote, that the shape of particles is spherical and regular, which can be concluded from SEM pictures. It’s not true. I think that it can’t be concluded from that pictures. Microphotographs have shown some irregular granulate. The shape should be described in a correct way; maybe some additional researches are necessary, such as TEM or HR SEM. Generally, all these micrographs can’t be a confirmation of the regular spherical shape of particles.
2. The micro images can’t illustrate that the Bi2O3 grains were embedded along with CoFe2O4 particles into the nanocomposite matrix. Such illustration can give only EDX map.
3. The quality of figure 3 is unacceptable. Graphs b and d do not fit on the page. The axis descriptions in this figure are illegible and should be corrected.
4. Equations on page 12 are shifted and hence illegible.
5. Table S1 presents absorbance values of dye solutions charged by various as-prepared photocatalysts during photodegradation experiment should be placed in the text of the publication in the results part.

I recommend reconsidering the manuscript after a major revision.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The authors have reported a study on the characteristics and photodegradation activity of nanocomposites of bismuth oxide and cobalt ferrite, synthesized using an approach of hydrothermal/solid-state reaction as a function of Bi₂O₃/CoFe₂O₄ ratio. The resulting nanomaterials have been characterized by XRD-EDX spectroscopic, SEM, UV/Vis DRS, and photodegradation activity measurements. However, the approach is not that new, lacks a lot of data or analysis, and there are some clarifications needed to be addressed throughout the manuscript. As this manuscript does not meet the standard of this journal, this work is not recommended for publication. In order for the author to make further revisions and find other proper journals, other comments are listed as follows.

1. In Fig. 1, it is recommended to add patterns of JCPDS 01-077-0426 and JCPDS 00-022-0515 for comparison.

2. Page 8, the cited reference No. 36 only shows that Fe and Co doping inhibited the growth of particles, and the design of surface properties and surface area was very important to obtain better photocatalytic activity. It does not mention the concepts or narratives related to spherical shape and other morphology. Therefore, it is strongly recommended to add the data of the specific surface area of the samples for further confirmation.

3. Figure 3, it is recommended that the clarity of the coordinates in the insets should be improved. And some parts of the Fig. 3(b and d) are cropped.

4. Page 11, no figure 7 or figure 8 (should be an illustration of the photodegradation scheme) can be found in the manuscript.

5. It is strongly recommended to add analyses of XPS (X-ray photoelectron spectroscopy) and PL (photoluminescence) to further discuss the mechanism of photodegradation in this system.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The work is about the photochemical degradation capacity of a catalyst on dyes in aqueous media. The catalyst is a physical mixture of two nanoparticles of different metal oxides. However, in my opinion, there are some aspects that need to be studied, reviewed and corrected for publication. In some sections, more clear information is missing.

The description of the photocatalytic degradation section (2.4) is incomplete and inaccurate. There are a lot of information about the reaction parameters that have been omitted. For instance:

• What was the temperature of the reaction experiments? Did the temperature remain constant or did the radiation cause the reaction medium to heat up? Was there any system to control the reaction temperature? All of these issues are important when trying to show kinetic results, due to the effect that temperature has on them.
• Was the initial concentration of dye used in the normal range present in the natural environment?
• They assume correlation between dye concentration and absorbance at a given wavelength, but not showed. What kind of correlation exists in their range of concentration?
• They indicate that sorption equilibrium is reached after 120 minutes, but do not provide additional information about it. Is this time enough for all samples? If the samples have different adsorption capacities, the initial concentration of each experiment will be different
• Water bodies existing in natural environments have specific properties (salinity, conductivity, alkalinity, pH, etc.). Nothing is indicated about these properties in the sample used and they certainly have a considerable impact on the reaction step.

I think that there is an important lack of information to understand and justify the studies presented later. Without minimizing the effort made, there is a lot of additional information that would need to be studied (only 5 degradation experiments) in order to reach the claimed conclusions.

It is not clear from Figure 4 if the values at zero time are due to the previous adsorption process, which has taken two hours. Nor is it understandable that the 1-hour time data has been omitted, since it is a value that appears in the supplementary material. Anyway, given the different adsorption capacity shown, the initial concentrations are not equal and in kinetic studies, where the reaction rate is concentration dependent, the analysis of the final concentration is not the most appropriate, being much better the determination of the kinetic constant of the process.

Another surprising aspect is reflected in Figure 5. Apart from the fact that it is not explained how the Ct/C0 ratio is determined, this figure shows how most of the total degradation takes place in the first two hours, more than 65%. However, according to Table S1 and Figure 4, the adsorption reduction during those two hours is very small, no more than 10%.

Furthermore, following the explanation given in the document for the calculation of the degradation efficiency together with the data shown in Table S1, the results shown in Figure 6 are incorrect.

In the opinion of this reviewer, it is surprising that, with only 5 experiments, a kinetic mechanism of reaction can be given. Does this radical mechanism explain the evolution shown in Figure 4? Radicals typically have very low half-lives, while the induction period observed is relatively long. Could the evolution shown in Figure 4 be due to an increase in temperature, due to radiation maintained for 5 hours, and that this increase in temperature caused an increase in the reaction rate?

In conclusion, in my opinion there are many unresolved unknowns in the presented work, together with the aforementioned errors, make it inadvisable to publish this work.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

The paper was revised according to the journal rules. No earlier study has been reported in literature regarding synthesis of Bi2O3/CoFe2O4 nanocomposite and its dye degradation potential.

Few revisions are required and they are reported below:

• please try to reduce acronyms from the abstract
• please try to check subscripts and superscripts
• please add references in "The traditional...pollutants"
• the transferring of dye pollutants to other organic compounds how can be treated? add few considerations
• check and add all details for the instruments used and added in section 2
• in section 3.1 please improve the quality of fig.1
• figure 3 should be revised, it is difficult to read
• I suggest to use the same style for all figures and tables
• the proposed mechanism should be revised

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I accept the received explanations and introduced corrections, but I still have reservations about the graphic form of publication. I recommend accepting the article after a few minor corrections, as listed below:

1. Figures 3 and 4 do not fit on the page.
2. Equations on page 12 are still shifted and hence illegible.

The publication should be improved in terms of graphics.

Author Response

"Please see the attachment."

Author Response File: Author Response.docx

Reviewer 2 Report

In “Author Response”, the authors have addressed most of the comments. However, the PL analysis can further provide the direct evidence for the suppression of electron-hole recombination mechanism explaining the enhancement of photocatalytic performance. Suggest that it is more appropriate to add PL data for future publication.

In addition, in Fig. 8, based on the standard hydrogen electrode (SHE) (or NHE), it is strongly recommended to indicate the potentials of each substance's VB and CB, and the potentials of O₂/•O₂^- and OH^-/•OH. 

Author Response

"Please see the attachment."

Author Response File: Author Response.docx