Photocatalytic Degradation of Toxic Dyes on Cu and Al Co-Doped ZnO Nanostructured Films: A Comparative Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments

This study prepared Cu and Al co-doped ZnO nanostructured thin films using a sol-gel method, systematically investigating the influence of doping ratios on crystal structures, morphologies, and photocatalytic degradation of toxic dyes. Additionally, it analyzed the correlation between structural changes and catalytic activity mechanisms. In my view, the following issues need to be addressed before consideration for publication：

1. 1. In Figure 2, several magnified inset SEM images were incorrectly labeled as "Fig. 4b and Fig. 4e" in the main text, which should be corrected to "Fig. 2b and Fig. 2e."
2. 2. In Figure 5a, the spelling of the vertical axis label "Intensity" is incorrect.
3. In the XPS analysis, the authors observed inconsistent shifts in Zn 2p_3/2binding energy with varying doping concentrations. Could you provide a clearer explanation for this phenomenon?
4. The authors inferred that copper was doped into the ZnO lattice predominantly as Cu⁺rather than Cu²⁺. Could you explain why Cu⁺formation is favored over Cu²⁺ in this system?
5. Could you explain why the photocatalytic degradation rate is rapid initially but significantly slows down in the later stages of the reaction?

 

 

 

 

 

 

 

 

 

 

 

Author Response

This study prepared Cu and Al co-doped ZnO nanostructured thin films using a sol-gel method, systematically investigating the influence of doping ratios on crystal structures, morphologies, and photocatalytic degradation of toxic dyes. Additionally, it analyzed the correlation between structural changes and catalytic activity mechanisms. In my view, the following issues need to be addressed before consideration for publication：

1. In Figure 2, several magnified inset SEM images were incorrectly labeled as "Fig. 4b and Fig. 4e" in the main text, which should be corrected to "Fig. 2b and Fig. 2e."

We thank the reviewer for a high crediting of our work. We have corrected the above mentioned mistypes.

2. In Figure 5a, the spelling of the vertical axis label "Intensity" is incorrect.

We have corrected the above mentioned mistype.

3. In the XPS analysis, the authors observed inconsistent shifts in Zn 2p3/2binding energy with varying doping concentrations. Could you provide a clearer explanation for this phenomenon?

Just as in the case of the analysis of experimental XRD data, we assume that the non-monotonicity of this shift is due to the difference in the atomic radii of copper and aluminum, which, being embedded in the crystal lattice and essentially being point defects, change the local electron environment of zinc atoms. Another reason responsible for this shift is the difference in the oxidation state of zinc, aluminum, and copper (2+ vs 3+ vs 1+), which can also cause the observed phenomenon when incorporated into the ZnO crystal lattice. We have made the appropriate additions to the text of the paper (line 324-330).

4. The authors inferred that copper was doped into the ZnO lattice predominantly as Cu⁺rather than Cu²⁺. Could you explain why Cu⁺formation is favored over Cu²⁺ in this system?

The presence of copper in the samples with an oxidation state of 1+ instead of the expected 2+ is most likely associated with the process of its reduction, which may be caused by reactions with carbon (the concentration of which in the samples exceeds 12 at.%), as well as by redistribution of the main charge carriers (electrons) in the sample, since their concentration increases significantly due to doping with aluminum. We have made the corresponding addition to the text of the paper (line 347-351).

5. Could you explain why the photocatalytic degradation rate is rapid initially but significantly slows down in the later stages of the reaction?

A slight slowdown in the photocatalytic decomposition of methyl orange under the influence of visible radiation, observed in cases of samples 1 and 3, may be associated with a difference in the kinetics of the reactions occurring from the pseudo-first order. For example, due to partial evaporation of water (the MO curve under visible light in Figure 10, d) under the influence of thermal radiation from the light source.

Reviewer 2 Report

Comments and Suggestions for Authors

The research presented by Yakushova et al. is very interesting, however a few comments for further improving the content of the manuscript are:

1. Introduction: Why the total concentration of co-dopants of 6 at.% has been selected? Why methyl orange has been selected as the model dye?
2. Materials and Methods: It would be better for the readers this section to be divided into sub-sections (e.g., 2.1. Synthesis of Cu,Al co-doped ZnO nanostructured films, 2.2. Characterisation methods, etc.).
3. A schematic illustration of the synthetic procedure should be added.
4. Line 79: H2O should be replaced with H₂O.
5. Regarding the conditions of the XRD measurements (e.g. mA, kV, step) should be added.
6. Line 129: Why this concentration has been selected?
7. Figure 1 should be replaced with one of higher resolution. In addition, the peaks of the corresponding JCPDS card should be added.
8. Which peaks have been used for the calculation of the average size through the Scherrer equation? Have the appropriate corrections regarding the instrumental broadening been made?
9. Lines 160-166 and 170-176 describe exactly the same. Please correct.
10. Figures 3-9 should be replaced with ones of higher resolution.
11. In the photocatalytic study, the photolysis of the dye, as well as the degradation under dark should be included.
12. BET analysis should be added in order to supplement the observed photocatalytic results.
13. Results section should be also divided into sub-sections.
14. References must be corrected based on the journal's template.
15.  

Author Response

The research presented by Yakushova et al. is very interesting, however a few comments for further improving the content of the manuscript are:

1. Introduction: Why the total concentration of co-dopants of 6 at.% has been selected? Why methyl orange has been selected as the model dye?

The authors thank the reviewer for a high credit of our results and for the reviewer’s comments. The total concentration of co-dopants is selected in such a way that, on the one hand, it is not too high so that it provided co-dopant incorporation into the zinc oxide crystal lattice without separating as separate metallic phases. On the other hand, the co-dopant concentration would ensure a significant change in the properties of the material. The choice of methyl orange as the analyzed organic pollutant is due, for the first reason, to its known toxicity at high concentrations in the aquatic environment. And for another reason, it is a classic model pollutant, which allows comparison of the results of photocatalytic activity of various wide-bandgap semiconductor oxides (TiO₂, ZnO, SnO₂, etc.). We have made the appropriate additions to the text of the paper (line 146-150).

2. Materials and Methods: It would be better for the readers this section to be divided into sub-sections (e.g., 2.1. Synthesis of Cu,Al co-doped ZnO nanostructured films, 2.2. Characterisation methods, etc.).

The structure of the article has been revised and divided into sections to make it easier for the reader to understand.

3. A schematic illustration of the synthetic procedure should be added.

We have added a generalized synthesis scheme into Figure 1.

4. Line 79: H2O should be replaced with H₂O.

We have corrected the above mentioned mistype.

5. Regarding the conditions of the XRD measurements (e.g. mA, kV, step) should be added.

We have specified the conditions of XRD measurements of the samples in the text of the article. The measurements were carried out in the angle range from 15° to 95°, the angular resolution was 0.05°, the scanning step was 1° per minute.

6. Line 129: Why this concentration has been selected?

The choice of concentration of methyl orange is determined by two main considerations. First, at a given concentration in an aquatic environment, methyl orange exhibits a certain degree of toxicity. Secondly, at 40 μmol/L its aqueous solution has a non-zero value of light transmittance coefficient in the spectral range of 300-600 nm, which allows us to estimate the rate constant of photocatalytic decomposition quite accurately. We have made the appropriate explanations to the text of the paper (line 88-91).

7. Figure 1 should be replaced with one of higher resolution. In addition, the peaks of the corresponding JCPDS card should be added.

We have changed the resolution of the image and added data on JCPDS card No. 36-1451.

8. Which peaks have been used for the calculation of the average size through the Scherrer equation? Have the appropriate corrections regarding the instrumental broadening been made?

When assessing the average crystallite size, peaks (100), (002) and (101) were used, for each of which the value of “di” was estimated (where i=1, 2, 3 corresponds to the peak number). After that this value was averaged over all three peaks in order to minimize the instrumental error. We have introduced the corresponding corrections to the text of the paper (line 190-193).

9. Lines 160-166 and 170-176 describe exactly the same. Please correct.

Thank you for finding this mistype. We have removed the duplicate.

10. Figures 3-9 should be replaced with ones of higher resolution.

We have improved the resolution of the figures

11. In the photocatalytic study, the photolysis of the dye, as well as the degradation under dark should be included.

We have made appropriate additions to the article, demonstrating the change in the concentration of methyl orange under the influence of UV radiation or visible radiation in the absence of photocatalytic material. In order to compensate for the effects associated with possible adsorption of the dye on the surface of the samples, as well as its degradation in the dark, the samples were placed in an aqueous solution of methyl orange and kept in the dark for 30 minutes before exposure to radiation. We have introduced the corresponding additions to the text of the paper (line 168-171).

12. BET analysis should be added in order to supplement the observed photocatalytic results.

Since our samples are nanostructured films on the surface of glass substrates, their study by the BET method is very difficult and often gives incorrect estimates.

13. Results section should be also divided into sub-sections.

The structure of the article has been revised and divided into sections to make it easier for the reader to understand.

14. References must be corrected based on the journal's template.

The references are reedited in accordance with the journal style.

Reviewer 3 Report

Comments and Suggestions for Authors

In this manuscript, the authors report nanostructured ZnO films was co-doped with Cu and Al to evaluate photocatalytic efficiency in decomposition of toxic dye (methyl orange) under UV and Vis radiation. Although the experimental work is systematically presented, However, there are some questions and unclear statements which must be answered and revised:

1. The authors thought that aluminum and copper were embedded in the ZnO crystal lattice. However, any shift of 2θ degree was not observed in XRD diffraction peaks of nanostructure ZnO:Al:Cu with the increase in almuminum concentration. The authors explain this. Generally, the shift of this value represents a change in the crystal structure of ZnO.
2. The intensity of Al2s was decreased in XPS spectra with the increase in aluminum concentration. The authors should explain the reason. Specially, the peak of Al2s was almost not observed in XPS spectra of sample 3 and 5.
3. Photocatalytic degradation kinetics of toxic dyes were studied by pseudo-first order reaction. How about the other kinetic reaction (pseudo-second order reaction)?
4. The authors only choose MO as photocatalytic decomposition. The other toxic dyes (such as methyl orange) should be also used to evaluate photocatalytic performance.
5. Photocatalytic mechanism should be provided?
6. The style of references should be carefully revised according to the requirements of journal.

Author Response

In this manuscript, the authors report nanostructured ZnO films was co-doped with Cu and Al to evaluate photocatalytic efficiency in decomposition of toxic dye (methyl orange) under UV and Vis radiation. Although the experimental work is systematically presented, However, there are some questions and unclear statements which must be answered and revised:

1. The authors thought that aluminum and copper were embedded in the ZnO crystal lattice. However, any shift of 2θ degree was not observed in XRD diffraction peaks of nanostructure ZnO:Al:Cu with the increase in almuminum concentration. The authors explain this. Generally, the shift of this value represents a change in the crystal structure of ZnO.

We thank the reviewer for a high credit of our work. We have clarified this point in the article in more detail. In particular, in Figure 2b we have added an insert showing that a shift of the 2θ angle is observed for the diffraction peak (002). The magnitude of this shift is maximum for samples 2 and 4 and small for the remaining samples 1, 3 and 5, which is explained by both the small total concentration of co-doping agents equal to 6 at.% and by the difference in their atomic radii. Since the atomic radius of Cu is 0.128 nm and is smaller than that of Zn, for which it is 0.138 nm, and for Al, in turn, the atomic radius is the largest and is 0.143 nm, the shifts were observed both towards larger and towards smaller values of 2θ, and can even balance each other.

2. The intensity of Al2s was decreased in XPS spectra with the increase in aluminum concentration. The authors should explain the reason. Specially, the peak of Al2s was almost not observed in XPS spectra of sample 3 and 5.

Since aluminum is identified by XPS spectra only for samples 2 and 4 and it is not present in samples 1, 3 and 5, it can be assumed that Al is contained mainly in the film volume, its content on the surface is under the limit of sensitivity of the XPS method. This assumption is indirectly confirmed by the data on the surface morphology of the samples, which in the case of samples 1, 3 and 5 do not contain pores which could provide the exit of electrons from the core levels of Al from the film volume.

3. Photocatalytic degradation kinetics of toxic dyes were studied by pseudo-first order reaction. How about the other kinetic reaction (pseudo-second order reaction)?

The second or higher order of reactions does not reliably describe the analyzed kinetic curves. In particular, when plotting them in the coordinates 1/C vs t, no linear dependence is observed. In the analyzed case, a fractional reaction order with the exponent degree “n” in the range from 1 to 2 is possible, however, for the overwhelming majority of samples “n” is very close to 1, based on which it seems appropriate to use pseudo-first order.

4. The authors only choose MO as photocatalytic decomposition. The other toxic dyes (such as methyl orange) should be also used to evaluate photocatalytic performance.

The choice of methyl orange as the analyzed organic pollutant is due to its known toxicity at high concentrations in the aquatic environment, on the one hand.  And on the other hand, it is a classic model pollutant, which allows comparison of the results of photocatalytic activity of various wide-bandgap semiconductor oxides (TiO2, ZnO, SnO2, etc.). Since the main goal of our article was to establish a correlation between the composition, crystal structure and surface morphology of nanostructured ZnO:Al:Cu films (with a total concentration of co-dopants of 6 at.%), in our opinion, it is advisable to conduct an analysis of other dyes in subsequent further studies.

5. Photocatalytic mechanism should be provided?

The mechanism of photocatalytic activity of zinc oxide doped with copper or aluminum was considered in detail in the framework of papers [46 and 47] and in the case of our paper the mechanism is a generalization of those suggested in refs 46 and 47. We have added a corresponding supplement to the text of the article (line 458-461).

46. Zhang, X.; Chen, Y.; Zhang, S.; Qiu, C. High Photocatalytic Performance of High Concentration Al-Doped ZnO Nanoparticles. Separation and Purification Technology 2017, 172, 236–24.
47. Chandekar, K.V.; Shkir, Mohd.; Al-Shehri, B.M.; AlFaify, S.; Halor, R.G.; Khan, A.; Al-Namshah, K.S.; Hamdy, M.S. Visible Light Sensitive Cu Doped ZnO: Facile Synthesis, Characterization and High Photocatalytic Response. Materials Characterization 2020, 165, 110387.

      6.The style of references should be carefully revised according to the requirements of journal.

The reference list is corrected according to the journal style.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

ACCEPT

Reviewer 2 Report

Comments and Suggestions for Authors

All suggested improvements and corrections have been done and they are supported by the revised manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

I would like to recommend the revised manuscript for publishing in journal.