Rapid Degradation of Organic Dyes by Nanostructured Gd₂O₃ Microspheres

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The author utilized a coprecipitation method to synthesize hollow porous Gd₂O₃ microspheres, which exhibited good efficiencies for Congo red (CR) and malachite green (MD) dye removal. This manuscript is overall well written, and the experimental results are interesting. However, further experiments are necessary to consolidate the statements and conclusions in this work before publication. Therefore, I recommend reconsidering the publication of this manuscript in Applied Nano after major revisions.

Specific comments are listed below:

(1)   In this manuscript, the author claimed the formation of hydroxyl radicals and other ROS species with Gd₂O₃ for the CR degradation. Is there any solid evidence for the existence of these ROS species and their participation in the CR degradation process?

(2)   For Figure 6b, please provide the cycling test of Gd₂O₃ for the degradation of CR in the dark and with light excitation.

(3)   For SEM and TEM images in Figure 3, the scale bars are missing. Please provide them.

(4)   In Figure 4c, the XPS fitting of Gd 4d peaks is terrible, which does not match the raw data at all. Please redo it with reasonable fittings.

(5)   For XPS results from Figure 4, since the author claimed the existence of carbonaceous residues and absorbed CO₂ on Gd₂O₃, I am wondering why the author did not fit the O 1s peaks from carbonaceous residues and absorbed CO₂ in Figure 4d, with only O 1s peaks from Gd-O and OH^-?

(6)   For Figure 5, Please also provide the original UV-vis absorption spectrum of Gd₂O₃ with wavelength as X axis and absorption as Y axis.

(7)   For XPS and FTIR spectra of samples after degradation, how did the author process the samples after degradation experiments before XPS and FTIR? The water and organic residues on sample surface might contribute to the increase of O-H peak in FTIR and O 1s peak of OH in XPS. In addition, in line 267, the increase of these peaks can not provide the evidence for the formation of hydroxyl radicals since these peaks do not necessarily correspond to hydroxyl radicals

Author Response

The author utilized a coprecipitation method to synthesize hollow porous Gd₂O₃ microspheres, which exhibited good efficiencies for Congo red (CR) and malachite green (MD) dye removal. This manuscript is overall well written, and the experimental results are interesting. However, further experiments are necessary to consolidate the statements and conclusions in this work before publication. Therefore, I recommend reconsidering the publication of this manuscript in Applied Nano after major revisions.

Specific comments are listed below:

• In this manuscript, the author claimed the formation of hydroxyl radicals and other ROS species with Gd₂O₃ for the CR degradation. Is there any solid evidence for the existence of these ROS species and their participation in the CR degradation process?

Response:

I appreciate the time spent reviewing this manuscript. It is an important question, however, is difficult to provide evidence on the formation of ROS. In this version, the paragraph was corrected and a short explanation about how this topic was managed can be found in line 269 and following, and a new reference [Nosaka].

• For Figure 6b, please provide the cycling test of Gd₂O₃ for the degradation of CR in the dark and with light excitation.

Please find the cycling test graph (Figure 6(c)) obtained from photocatalytic degradation experiments, with the corresponding explanation. Since it has similar characteristics than that corresponding to adsorption tests the latter was included in supplementary material.

• For SEM and TEM images in Figure 3, the scale bars are missing. Please provide them.

Sorry for the omission, something went wrong in the previous submission. They now appear in each photo

• In Figure 4c, the XPS fitting of Gd 4d peaks is terrible, which does not match the raw data at all. Please redo it with reasonable fittings.

Please find in this version an improved version of this figure, it was made with the Originlab software (with its limitations to process peaks). At this moment, better software such as XPSCasa for making XPS graphics is not available. Sorry for the inconvenience.

• For XPS results from Figure 4, since the author claimed the existence of carbonaceous residues and absorbed CO₂ on Gd₂O₃, I am wondering why the author did not fit the O 1s peaks from carbonaceous residues and absorbed CO₂in Figure 4d, with only O 1s peaks from Gd-O and OH^-?

Thanks for the observation, the expected peak corresponding to C^_O binding energy would be between 530.5 to 531.5 eV. It is difficult to locate this signal in this spectrum since apparently O-H bonds dominate in this spectral region.

• For Figure 5, Please also provide the original UV-vis absorption spectrum of Gd₂O₃ with wavelength as X axis and absorption as Y axis.

Please find the corresponding graph. It is possible to observe small differences in these graphs since these measurements were made several times, and those shown are some of the most representative.

• For XPS and FTIR spectra of samples after degradation, how did the author process the samples after degradation experiments before XPS and FTIR? The water and organic residues on sample surface might contribute to the increase of O-H peak in FTIR and O 1s peak of OH in XPS. In addition, in line 267, the increase of these peaks can not provide the evidence for the formation of hydroxyl radicals since these peaks do not necessarily correspond to hydroxyl radicals

After photocatalysis and adsorption experiments, the samples used in XPS and FTIR were dried at 100°C for 8 h. in a laboratory electric oven. Then, water contained in the samples by soaking them in the dye solution was removed. So, it is believed that it did not interfere or to a lesser extent. About the evidence of formation of OH, I agree with the comment, and it was corrected, as it was mentioned before.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents a novel method to synthesize gadolinium oxide and its application to degrade two selected organic dyes. The work can be accepted for publication after addressing the following points:

1.      During the synthesis, formic acid and pectin were used. It is important to describe their role in the synthesis process.

2.      Considering the synthesis process, after vigorous stirring (probably manual), is a maturation step involved? Also, please specify the microwave power and duration of the irradiation process.

3.      The XRD methodology lacks the step size and scanning speed to provide insight into resolution and data acquisition time. Sample preparation details, such as how the material was mounted (e.g., powder form on a holder or thin film on a substrate), are crucial for understanding the results.

4.      The photocatalytic measurements were obtained over 2 hours. Was stirring involved? Given that the graphs have error bars, how many replicates were taken? Was it the same experiment with multiple samples taken at the same time, or were different experiments performed with one sample each measured? What was the volume of each measured sample?

5.      What is the typical crystalline size of the gadolinium oxide after calcination at 600C?

6.      The microscopy images in Figure 3 lack the scale bars.

7.      Given the shape shown on the micrographs, the product is more likely gadolinium oxide nanoparticles forming microspheres. In addition, it would be important to compare the obtained morphology with the literature, please refer to doi.org/10.3390/cryst11091094

8.      Can you please show the numbers described in the text on the micrographs? Diameter, thickness, and such?

9.      Considering that gadolinium oxide during the photocatalytic process was suspended in water which also contains hydroxyl ions, it is not clear that the OH groups found in the sample after the catalysis are a result of ROS generation. The clear evidence might be obtained from Electron Spin Resonance (ESR) Spectroscopy.

10.   Please report the efficiency of gadolinium oxide and other tested dyes in the literature.

 

11.   It is important to clarify the reason why the organic dyes are difficult to remove with regards to their structure, such as  conjugated double bonds in the aromatic rings leading to electron delocalization with resonance stabilization, please refer to doi.org/10.3390/su16208758

Author Response

The paper presents a novel method to synthesize gadolinium oxide and its application to degrade two selected organic dyes. The work can be accepted for publication after addressing the following points:

1. During the synthesis, formic acid and pectin were used. It is important to describe their role in the synthesis process.

Response

Thanks for your time spent reviewing the manuscript. I agree with the comment, please find the corresponding explanation (including the formation reaction of gadolinium oxide) in section 3.1, it was highlighted in yellow.

 

2. Considering the synthesis process, after vigorous stirring (probably manual), is a maturation step involved? Also, please specify the microwave power and duration of the irradiation process.

It is very likely that a maturation process occurred from the precipitation of the gadolinium formate microspheres to the formation of the oxide, unfortunately I did not consider at that time taking samples at different periods since I focused on obtaining only the oxide.

Please find the requested information in section 2.

3. The XRD methodology lacks the step size and scanning speed to provide insight into resolution and data acquisition time. Sample preparation details, such as how the material was mounted (e.g., powder form on a holder or thin film on a substrate), are crucial for understanding the results.

Thanks for the comment, please find this information in section 2.

4. The photocatalytic measurements were obtained over 2 hours. Was stirring involved? Given that the graphs have error bars, how many replicates were taken? Was it the same experiment with multiple samples taken at the same time, or were different experiments performed with one sample each measured? What was the volume of each measured sample?

About the stirring of samples used in dye degradation, effectively, all the photocatalytic and adsorption experiments were done under stirring (it was added in the manuscript).

The experiments were done in triplicate, except for photolysis which was done only once for each solution. In each experiment, samples were withdrawn from the stirred solution at the indicated times. Only one experiment was done at a time from which 10 samples were taken. In total, approximately 80 samples were analyzed using different powder and a fresh solution in each set of samples.

5. What is the typical crystalline size of the gadolinium oxide after calcination at 600C?

It was 23.4 nm, and the grain size observed by TEM was approximately 80 nm. These values can be found in the manuscript.

6. The microscopy images in Figure 3 lack the scale bars.

Sorry for the omission, please find the microscopy images with corresponding scale bars

7. Given the shape shown on the micrographs, the product is more likely gadolinium oxide nanoparticles forming microspheres. In addition, it would be important to compare the obtained morphology with the literature, please refer to doi.org/10.3390/cryst11091094

Thanks for the suggestion, the article of Ortega-Berlanga, B. et al. was added in the references.

8. Can you please show the numbers described in the text on the micrographs? Diameter, thickness, and such?

Of course, please find the corresponding markers in the photographs.

9. Considering that gadolinium oxide during the photocatalytic process was suspended in water which also contains hydroxyl ions, it is not clear that the OH groups found in the sample after the catalysis are a result of ROS generation. The clear evidence might be obtained from Electron Spin Resonance (ESR) Spectroscopy.

I agree with this comment, section 3.4 was rewritten to clarify this point.

10. Please report the efficiency of gadolinium oxide and other tested dyes in the literature.

Thanks for the comment, please find it in the conclusions section.

11. It is important to clarify the reason why the organic dyes are difficult to remove with regards to their structure, such as  conjugated double bonds in the aromatic rings leading to electron delocalization with resonance stabilization, please refer to doi.org/10.3390/su16208758

Thanks for the interesting suggestion, please find the corresponding information at the end of section 3.3, and the reference of Sidorowicz et al.

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

The work entitled "Rapid Degradation of Organic Dyes by Nanostructured Gd2O3 Microspheres" presents the synthesis, characterization, and application of Gd2O3 microspheres in the degradation of dyes. While this study addresses a significant topic and presents numerous results, several details could be added to enhance its clarity and depth. Here are some suggestions:

1. In the abstract, replace the term “pigments” with “dyes,” as these are distinct terms. Additionally, replace “•O₂^- ions” with “superoxide radicals (•O₂^-).”

2. In the synthesis section of gadolinium oxide, clarify the roles of formic acid and pectin.

3. The actual contributions of photocatalysis and adsorption processes in dye removal are not clearly defined. Although two different experiments were conducted—one for each method—could there be a synergistic effect? Is there an initial adsorption phase prior to the photocatalytic process?

4. Given that the material has significant adsorption capacity, it would be valuable to include nitrogen adsorption/desorption analyses to evaluate the surface area and porosity. These results could greatly influence the photocatalytic and adsorptive activities of the material.

5. In calculating the band gap, what equation was used? Why do the values obtained in this work differ from those in the literature?

6. In the discussion regarding the zero charge point of gadolinium oxide and its interaction with dyes, was this based solely on literature, or was it experimentally verified?

7. In section 3.4, where XPS and FTIR analyses were performed to confirm the presence of hydroxyl and superoxide radicals in the dye degradation mechanism, it is unclear how these analyses substantiate this claim. The radicals in question are free radicals rather than functional groups added to the molecules. While there is likely an adsorption process involving the dyes on the semiconductor, potentially through electrostatic attraction, this does not necessarily indicate that oxygen radical species are responsible for dye degradation.

8. In the abstract, the author states that the •OH and •O₂^- radicals are responsible for dye degradation; however, in section 3.4, only a discussion of the •OH radicals is presented. How was the involvement of superoxide radicals determined?

 

These revisions aim to improve the overall clarity and comprehension of the work while addressing specific areas that require further elaboration.

Author Response

The work entitled "Rapid Degradation of Organic Dyes by Nanostructured Gd2O3 Microspheres" presents the synthesis, characterization, and application of Gd2O3 microspheres in the degradation of dyes. While this study addresses a significant topic and presents numerous results, several details could be added to enhance its clarity and depth. Here are some suggestions:

1. In the abstract, replace the term “pigments” with “dyes,” as these are distinct terms. Additionally, replace “•O₂^-ions” with “superoxide radicals (•O₂^-).”

Response

Thank you for your time spent reviewing the manuscript.

The correct terms have been placed throughout the manuscript

2. In the synthesis section of gadolinium oxide, clarify the roles of formic acid and pectin.

Please find this information in section 3.1 (description of SEM images)

 

3. The actual contributions of photocatalysis and adsorption processes in dye removal are not clearly defined. Although two different experiments were conducted—one for each method—could there be a synergistic effect? Is there an initial adsorption phase prior to the photocatalytic process?

Thanks for providing this observation, a synergy between adsorption and photodegradation is highly probable (under exposure to UV light). In the absence of radiation, what was found in the literature for rare-earth oxides (in articles like those cited below), was the predominance of dissociative adsorption. This is due to the bandgap energy of most of these materials is larger than the photon energy of common UV lamps of enough optical irradiance. Moreover, during the first experiments discoloration of dye solutions was observed in samples where the UV LED was not used, then it was assumed that dissociative adsorption played a key role.

• Mitchell, M.B.; Sheinker, V.N.; Mintz, E.A. Adsorption and Decomposition of Dimethyl Methylphosphonate on Metal Oxides, J. Phys. Chem. B 1997, 101, 11192-11203.
• O. Gordon, et al. Adsorption and Decomposition of Dimethyl Methylphosphonate on Y₂O₃ Nanoparticles, J. Phys. Chem. C 2007, 111, 3233-3240

4. Given that the material has significant adsorption capacity, it would be valuable to include nitrogen adsorption/desorption analyses to evaluate the surface area and porosity. These results could greatly influence the photocatalytic and adsorptive activities of the material.

I agree with this recommendation, it will give valuable information about the surface area of Gd₂O₃ microspheres. Unfortunately, finding the right instrument to perform this task will take a longer time than that indicated by the MDPI editorial team. The shortage of scientific instruments and the lack of maintenance has been severe lately, but once I have the results, I can add them in a repository or supplementary material file.   

 

5. In calculating the band gap, what equation was used? Why do the values obtained in this work differ from those in the literature?

To obtain the bandgap energy of Gd₂O₃, from UV-vis spectroscopy data, the relationship used was: (αhν)² vs. hν, which appears in line 219. This is considering the oxide as a direct transition semiconductor. In this expression α is the optical absorption coefficient and hν the photon energy. The difference with what is reported in the literature may be attributed to slight hydration, although samples were always kept into a desiccator with silica gel.

6. In the discussion regarding the zero charge point of gadolinium oxide and its interaction with dyes, was this based solely on literature, or was it experimentally verified?

It was based on literature; it was not measured on the samples. Further characterization is needed to confirm it.

7. In section 3.4, where XPS and FTIR analyses were performed to confirm the presence of hydroxyl and superoxide radicals in the dye degradation mechanism, it is unclear how these analyses substantiate this claim. The radicals in question are free radicals rather than functional groups added to the molecules. While there is likely an adsorption process involving the dyes on the semiconductor, potentially through electrostatic attraction, this does not necessarily indicate that oxygen radical species are responsible for dye degradation.

I agree with this observation. This part was revised, and the new explanation appears in section 3.4

8. In the abstract, the author states that the •OH and •O₂^-radicals are responsible for dye degradation; however, in section 3.4, only a discussion of the •OH radicals is presented. How was the involvement of superoxide radicals determined?

It is an important point, but difficult to verify experimentally. Since these radicals have a very short life, their detection becomes a major challenge. They are mentioned in the abstract based on the literature, but experimental evidence needs more, with the drawback that this oxide is highly hygroscopic.

These revisions aim to improve the overall clarity and comprehension of the work while addressing specific areas that require further elaboration.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Required experimental data has been added and the authors have solved most of the concerning issues in this manuscript. Thus, I recommend considering the publication of this manuscript in Applied Nano in present form.

Author Response

Required experimental data has been added and the authors have solved most of the concerning issues in this manuscript. Thus, I recommend considering the publication of this manuscript in Applied Nano in present form.

Response:

The time spent reviewing the manuscript is greatly appreciated

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript was improved based on the provided suggestions. However, one aspect remains development. Given that the main role of the synthesized material is degradation of organic pollutants, it is important to make sure that introduction of the material will not cause any potential contamination. Thus, the recovery and re-usability of the material should be discussed.

Author Response

The manuscript was improved based on the provided suggestions. However, one aspect remains development. Given that the main role of the synthesized material is degradation of organic pollutants, it is important to make sure that introduction of the material will not cause any potential contamination. Thus, the recovery and re-usability of the material should be discussed.

Response:

Thanks for the comment, I agree with that. Please find a paragraph concerning this topic in lines 265 and the following and the corresponding references that support this information.