Mixed Oxides NiO/ZnO/Al₂O₃ Synthesized in a Single Step via Ultrasonic Spray Pyrolysis (USP) Method

Round 1

Reviewer 1 Report

In this manuscript, the authors reported on the synthesis of nanostructured mixed oxides composite "NiO/ZnO/Al₂O₃" system. The focus is on the growth and the structural/morphological characterization of the composite. The idea is original that consist of mixing the three oxides to form a nano-composites for possible applications. However, the authors didn’t characterize other applications-related properties such as "gas sensing", "catalysis", but focus only on structural analysis.

Nevertheless, the paper is still interesting as it present relatively new material prepared UPS. However, the authors should address all the raised concerns and are appended below. 

1. 
   Figure 1 is schematic of the setup. Is it original figure of the authors ?. The authors need to improve it.
2. The resolution of Figure 3 needs really to be enhanced.
3. In the statement (lines 228-229), the authors should support their claim of the existence of nano-sized particles by analysis of the SEM images for the grains/particles size.
4. How the authors ruled-out the existence of ZnO and NiO phase from XRD spectra? If they tested the spectra with JCPDS files of ZnO and NiO pure phases it should be mentioned and commented.
5. Which XRD software has been used to assign the phases ?
6. In Figure 4, XRD peaks are more resolved at 400 and better at 600C, but at 800C the XRD peak intensity is weak (close to background). The authors need to explain this and add their explanation in the text?
7. in figure 4b) there are 2 XRD peaks not assigned at 52º and 56º? What could be the origin of these peaks?
8. The Raman spectra seems to be interesting but are not discussed fully in the text. For example, the effect of the temperature is not discussed. Why the peak of NiO increases strongly at 600C and also 800C?
9. There seems to be a contradictions between Raman and XRD results. The XRD results excluded the existence of the pure phases of ZnO and NiO, but the Raman spectra show clearly the existence of these two oxides. The authors need to explain this in the text.
10. The authors in lines 253-255 attributes some Raman peaks to Al2O3 and ZnO vibrations but no references is added to justify this.
11. The overall discussion of the Raman results need to be changed and improved.
12. In the conclusion, the authors stated that the optimum temperature of 600C (line 269). I failed to in the manuscript a discussion on the effect of temperature that support this conclusion. If this is the case, the authors need to add paragraphs in the manuscript that support this interesting conclusion.

Author Response

Dear Reviewer, thank you very much for your invested time and valuable comments. In this manuscript, the authors reported on the synthesis of nanostructured mixed oxides composite "NiO/ZnO/Al₂O₃" system. The focus is on the growth and the structural/morphological characterization of the composite. The idea is original that consist of mixing the three oxides to form a nano-composites for possible applications. However, the authors didn’t characterize other applications-related properties such as "gas sensing", "catalysis", but focus only on structural analysis.

Nevertheless, the paper is still interesting as it present relatively new material prepared UPS. However, the authors should address all the raised concerns and are appended below. 

1. Figure 1 is schematic of the setup. Is it original figure of the authors? The authors need to improve it.

            Figure 1 is the original drawing of our experimental setup. However, since we can not further develop the schematic of the setup, we have added an image of our own experimental setup.

2. The resolution of Figure 3 needs really to be enhanced.

            The resolution was degraded because the figure size was reduced. Editing was done by increasing the size of the figure.

3. In the statement (lines 228-229), the authors should support their claim of the existence of nano-sized particles by analysis of the SEM images for the grains/particles size.

            Calculation was made from the SEM analysis results with ImageJ program. And the results confirmed that the primary particles are nanometer in size.

            Max. = 26.84 nm

            Min. = 4.11 nm

            Average = 8.65 nm

Reference: Schneider, C. A., Rasband, W. S., Eliceiri, K. W. NIH Image to ImageJ: 25 years of image analysis. Nature Methods, 2012, 9(7), 671–675. doi:10.1038/nmeth.2089.

4. How the authors ruled-out the existence of ZnO and NiO phase from XRD spectra? If they tested the spectra with JCPDS files of ZnO and NiO pure phases it should be mentioned and commented.

            The diffraction peaks of the particles matched well with the diffraction data from the JCPDS card no. 10–0339 (NiAl₂O₄) and JCPDS card no. 00-005-0669 (ZnAl₂O₄). For NiAl₂O₄ and ZnAl₂O₄ samples no peaks characteristics of NiO and ZnO are seen indicating fine dispersion of these species on the NiAl₂O₄ and ZnAl₂O₄ supports respectively or may be overlapped with the supports difraction peaks.

            The peaks (2θ of 31.77° and 34.42°) in the XRD pattern of ZnO (JCPDS card no. 00-036-1451) were not observed in our structure. Only the peak at 36.2° is compatible, but that peak is also compatible with ZnAl₂O₄.

            In fact, the reason for this is in the article. Thermodynamic analyzes show that the structure is converted to aluminates (in lines 185-190).

5. Which XRD software has been used to assign the phases?

            X'Pert Highscore Plus has been used to assign the phases.

6. In Figure 4, XRD peaks are more resolved at 400 and better at 600C, but at 800C the XRD peak intensity is weak (close to background). The authors need to explain this and add their explanation in the text?

XRD analysis of the sample belonging to 800 °C was done in a different device. Therefore, a difference was observed in the background. However, it has peaks belonging to the NiAl₂O₄ and ZnAl₂O₄ phases.

7. in figure 4b) there are 2 XRD peaks not assigned at 52º and 56º? What could be the origin of these peaks?

            The peaks were re-examined and found to belong to ZnAl₂O₄ and NiAl₂O₄. Figure 4b was redrawn by making necessary corrections.

8. The Raman spectra seems to be interesting but are not discussed fully in the text. For example, the effect of the temperature is not discussed. Why the peak of NiO increases strongly at 600C and also 800C?

            Increasing process temperature explains the order of magnitude increase in the Raman signal.

9. There seems to be a contradictions between Raman and XRD results. The XRD results excluded the existence of the pure phases of ZnO and NiO, but the Raman spectra show clearly the existence of these two oxides. The authors need to explain this in the text.

            Misnaming has been made in the description of the Raman analysis results. The detected raman peaks belong to the tetrahedral vibrations of NiAl₂O₄ and ZnAl₂O₄.

10.The authors in lines 253-255 attributes some Raman peaks to Al2O3 and ZnO vibrations but no references is added to justify this.

            Necessary arrangements were made for Raman analysis and added to the text.

11. The overall discussion of the Raman results need to be changed and improved.

            Necessary arrangements were made for Raman analysis and added to the text.

12. In the conclusion, the authors stated that the optimum temperature of 600C (line 269). I failed to in the manuscript a discussion on the effect of temperature that support this conclusion. If this is the case, the authors need to add paragraphs in the manuscript that support this interesting conclusion.

            The reason why 600 °C is the optimum temperature is explained in the conclusion.

I hope that our answers are sufficient to consider our paper for publishing in Metals.

Reviewer 2 Report

Authors described the Mixed Oxides NiO/ZnO/Al2O3 Synthesized in a Single Step via Ultrasonic Spray Pyrolysis (USP) Method. but the topic is not on Metals, which is out the scope of the journal. 

Thus, it is not acceptable for metals, but suitable for materials (MDPI), or inorganics (MDPI) and so on.

This paper studied the mixed oxides NiO/ZnO/Al2O3 were synthesized 14 in a single step via the ultrasonic spray pyrolysis method using nitrate salts and investigated of the 15 process temperature effects (400, 600, 800 °C). it can attract the readers in the quartz field and metallurgical field. After answering the following question,it can be published in MDPI journals except Metals:

1)Fig. 3 should be redrawed, since the labels and denotes are too unclear.

2)Fig.4c, the Al2O3 signal is too weak, compared with the background noise, but from the SEM images in Fig.5, 800 oC sample does not seem very specical, why?

3)Fig. 6, the NiO peak with 800 oC is very strong, and its NiAlO peak in Fig. 4 is not so strong. What is the reason for this mismatching?

4)Is there any TEM test for checking the products?

5)In conclusion, authors claim the nanocomposite particles, but the average size of the particles is 1 miron or more. Is it suitable to say nano-?

6)The format of reference should be unified.

Author Response

Dear Reviewer,

thank you very much for your invested time and Valuable comments. We are sending our answers on your remarks in bold letters.

Authors described the Mixed Oxides NiO/ZnO/Al2O3 Synthesized in a Single Step via Ultrasonic Spray Pyrolysis (USP) Method, but the topic is not on Metals, which is out the scope of the journal.

            Thus, it is not acceptable for metals, but suitable for materials (MDPI), or inorganics (MDPI) and so on.

            This paper studied the mixed oxides NiO/ZnO/Al2O3 were synthesized in a single step via the ultrasonic spray pyrolysis method using nitrate salts and investigated of the process temperature effects (400, 600, 800 °C). It can attract the readers in the quartz field and metallurgical field. After answering the following question,it can be published in MDPI journals except Metals:

Our special Issue "Advances in Understanding of  Unit Operations in Extractive Metallurgy of  Non-Ferrous Metals" is focused on different metallurgical operations for synthesis of metals, metal oxides and composites. Ultrasonic spray pyrolysis belongs to these operations.Therefore we included this paper in our previous mentioned special Issue, although your opinion is in contrast to our aim.

Our answers on your questions:

1)Fig. 3 should be re-drawed, since the labels and denotes are too unclear.

            The resolution was degraded because the figure size was reduced. Editing was done by increasing the size of the figure.

2)Fig.4c, the Al₂O₃ signal is too weak, compared with the background noise, but from the SEM images in Fig.5, 800 °C sample does not seem very specical, why?

            XRD analysis of the sample belonging to 800 °C was done in a different device. Therefore, a difference was observed in the background. Starting with the appearance of very irregular shape both spherical and foliated morphology is formed at 800 °C. As the temperature increased, the morphology also changed.

3)Fig. 6, the NiO peak with 800 °C is very strong, and its NiAlO peak in Fig. 4 is not so strong. What is the reason for this mismatching?

            Misnaming has been made in the description of the Raman analysis results. Raman analysis results were rearranged. The detected raman peaks belong to the tetrahedral vibrations of NiAl₂O₄ and ZnAl₂O₄. The Raman peak of spinels increases strongly at 600 °C and 800 °C and NiO vibrations are decreasing and disappearing.

4)Is there any TEM test for checking the products?

            Unfortunately, We don’t have TEM analysis, and there is no TEM-device in our institution. We also do not have samples for TEM analysis. We are also unable to repeat our experimental work due to the malfunction of the USP assembly. The repair of the experimental setup is delayed due to Covid-19. We hope that SEM analysis id sufficient to explain the obtained Morphology of particles.

5)In conclusion, authors claim the nanocomposite particles, but the average size of the particles is 1 micron or more. Is it suitable to say nano-?

            Calculation and measurement was made from the SEM analysis results with ImageJ program. And the results confirmed that the primary particles are nanometer in size.

            Max. = 26.84 nm

            Min. = 4.11 nm

            Average = 8.65 nm

Reference: Schneider, C. A., Rasband, W. S., Eliceiri, K. W. NIH Image to ImageJ: 25 years of image analysis. Nature Methods, 2012, 9(7), 671–675. doi:10.1038/nmeth.2089.

6)The format of reference should be unified.

            The reference has been rearranged in accordance with the format rules.

I hope that this improvement can fill strict request for the publishing in Metals.

Thank you very much for your participation in this procedure..

Round 2

Reviewer 2 Report

my comments are addressed successfully.

Author Response

Dear Reviewer,

thank you for important comments and invested time.

We will check this text and improve it.

Best regards

Srecko Stopic