Influence of Al Doping on the Morphological, Structural and Gas Sensing Properties of Electrochemically Deposited ZnO Films on Quartz Resonators

Round 1

Reviewer 1 Report

The manuscript by Tyutyundzhiev et al. describes the detection of ammonia using a sensor composed of Al-doped ZnO nanostructures on gold electrodes of quartz resonators. The authors probe the effect of Al on the morphology, gas-sensing property of the sensor. Overall, the manuscript is well written. I am quite intrigued by the findings of the papers and would recommend the paper for publication. However, there are a few concerns that need to be addressed. I'll list those concerns pointwise.

1. Lines 97-99: Under Materials and Methods, the authors mention ‘The thickness of the ZnO layers, the concentration of the dopant Al and the surface morphology were controlled by deposition potentials, temperature, Al2(SO4)3 concentration and plating time.’ Can the authors elaborate on the conditions?
2. Figure 3 needs to be better represented. The labels and the 3D images are impossible to read due to their petite size.
3. What is the working temperature for figure 5?
4. How does the concentration of gases depicted in figure 5 compare with the concentration of ammonia in the air under normal versus hazardous conditions? The maximum exposure level for NH₃ stated by OSHA is about 50 ppm. What is the sensitivity of the sensor at similar concentrations?
5. What is the response time for this sensor?
6. Lines 204-205: ‘The time depends on the conditions, temperature, and humidity but is about 24 hours.’ Can the authors elaborate on the conditions and other related parameters? Generally, a fast response and recovery time are desirable for such sensors.
7. Can the authors comment on the response of the sensor over various cycles?
8. Since the authors propose this electrochemical sensor as an NH₃ sensor, how does the sensor response vary in the presence of other gases?

 

Author Response

Thank you very much for the positive quality assessment and the questions helpful for article clearing and improvement.     

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments:

In this work, the deposition of aluminum-doped ZnO nanostructured thin films was done on gold electrode of quartz resonators. The electrodeposition is quite inexpensive technique which does not require complicated equipment and allows coating of large areas with controlled and patterned growth of diversе on morphology and size nanostructures. The approach presented in this work provides an effective way to study the morphological, structural and gas sensing properties of electrochemically ZnO films deposited at different concentrations of Al2(SO4)3 on quartz resonators.

In summary, the main ideas and mechanism of the enhancement in the Al-doped ZnO layers were conveyed straightforwardly. I think this manuscript is suitable to be published at this journal (Coatings) after addressing the following revisions.

1. The results were expected one, but the stated Δm results were not understandable. How did they relate the mass and frequency?
2. It will be abundant if the authors could describe the Sauerbrey equation in more detail, which could lead readers to a better understanding of the work.
3. The typesetting of figures needs to be improved. In Figure 4, the SEM image should have a scale.
4. There is no specific explanation for the effect of roughness on result proof in this work.
5. In page 6, “This is due to the fact that at lower concentrations we have a relatively small number of ammonia molecules that react with the layer and do not completely cover its surface.” Why did the sensitivity present such a trend? The relation of sensitivity and concentrations does not seem clear in Figure 5.
6. The applications of resulting products are not well reflected in this manuscript. It would be better if the authors discussed this.
7. The innovations in this paper focus on novel applications, and the methods and theoretical innovations are not sufficient. The author must dig deeper into the innovation points before the article can be accepted.

Author Response

Thank you for the positive assessment and the questions helpful for article clearing and improvement.     

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors investigated the influence of Al doping on morphological, structural and gas sensing properties of electrochemically deposited ZnO films on quartz resonators. The results are valid and interesting to the readers. However, the necessary references and physical mechanism are absent in the manuscript to explain the related results. In summary, I recommend the manuscript to undergo a minor revision to address my comments below before resubmission to this journal.

1. Line 58-69, it is written that “It is known that doping with Al is able to improve the sensitivity and conductivity of ZnO nanomaterials ...” Please clarify what difference if the Al is replaced by doping with other novel metal (for example Ti, Fe or other nanometal) and cite the related reference.

2. Typo, for example,

line 112, “with a volume of 22,5 liters,” should be “with a volume of 22.5 liters,”

Please check them. In the same manner, please check the typos throughout this manuscript.

3. Line 173, please show the Sauerbrey equation in the text.

4. Line 191-202, authors explained the mechanism of sensitivity. The sensing mechanism is suggested to be give in more detail and cite the related references.

Author Response

Thank you for the positive assessment.

Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript entitled “Influence of Al doping on morphological, structural and gas sensing properties of electrochemically deposited ZnO films on quartz resonators” The authors have characterized the sample extensively to achieve the properties which directly affect the VOCs sensing. However, a few more clarifications are needed before the manuscript can be considered suitable for publication. Novelty missing since similar work reported in (J. Phys.: Conf. Ser. 113 012044). Overall, the paper has many weaknesses that need to be overcome.  A list of other comments that need to be addressed follows:

 

Main comments

1. Abstract: important information missing, concentration range, response/recovery time, selectivity compared to other gases and long-term stability.
2. It is better to mention in the introduction how the selectivity could be enhanced? Include some strategies to improve selectivity and cite relevant references
3. Experimental part: ZnO and Al-ZNO synthesis details are missing?
4. XRD plot is missing? It is better to include a XRD plot of the ZnO and Al-ZnO.
5. How gas concentration been calibrated? Please explain in detail.
6. Some parameters are missing about the gas sensor measurement, like what is the SCCM of the gas? What is the applied voltage?
7. A gas sensing mechanism is missing, which is very important for Al-doped ZnO materials. Please discuss surface interaction details and use these articles as a reference: J. Phys.: Conf. Ser. 113 012044, Journal of Alloys and Compounds 748 (2018) 6-11, Microchim Acta 186, 418 (2019).
8. Selectivity with other reducing and oxidising gases are missing.
9. Please include a comparative table with other Al-ZnO-based sensors to understand the sensor results.
10. It is better to mention in the introduction how the selectivity could be enhanced? Include some strategies to improve selectivity and cite relevant references: Some suggested review articles to cite in the intro. Part: Adv. Mater. 2016, 28, 795–831, Multifunctional Materials 4, 032002 (2021).
11. Please include a gas sensing mechanism with the energy band diagram.
12. The long-term stability of the sensor is missing.
13. The quality of some figures is inferior and needs to be enhanced.
14. It is better to check and correct the font size of the x and y-axis of all figures in the manuscript. It should be the same.
15. The first paragraph contains trivial statements. The introduction should be reduced in length and focus on current analytical challenges. Essential related works can be cited.

Author Response

We would like to thank Reviewer 4 for the critical, helpful and interesting comments and suggestions.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Figure 4b-c are blank, which needs to be modified.

Author Response

Thanks for this comment, we've corrected the white boxes in Figure 4b-c, and removed some spelling inaccuracies from the text.

Reviewer 4 Report

All the comments are addressed properly, the manuscript is ready for the publication

Author Response

Thank you very much for appreciating the efforts we have made in correcting this publication.