Synthesis of Zinc-Titanium Oxide Nanocomposites by Plasma Jet and Its Application to Photocatalyst

Round 1

Reviewer 1 Report

The authors employed an atmospheric soft plasma jet and post-heat treatment process to synthesize Zn-Ti oxide nanocomposites. The nanostructures were well characterized and their photocatalyst application was also systematically studied. However, the designed nanostructures are not novel and do not show any better properties. The application in photocatalysis is somewhat a mere combination of old knowledge, but progress from the pre-existing systems is not so sufficient. I have not been completely convinced to recommend its acceptance in Catalysts. There are other specific points that the authors should address before attempting to resubmit this work to a more appropriate journal.

1. Compared with other synthesis strategies, what are the advantages of the plasma jet method for preparing bi-metal oxide nano-composites?

2. The authors should provide evidence for the element distribution of Zn and Ti.

3. The kinetics curves in figure 5 are clearly non-linear. Thus, it is not correct to fit the first-order kinetics. Would the reaction constants be calculated within shorter reaction times?

Author Response

Dear Reviewers,

Thank you very much for refereeing our paper and for giving us your kind corrections and suggestions as well as valuable comments. We have been accepted your fruitful comments, suggestions, and recommendations with all our heart and made our best to revise the manuscript (marked with red color for the corrected parts) with your advice. Thus, hereby, we prepared the answers for you as followings.

Reviewer 1

 The authors employed an atmospheric soft plasma jet and post-heat treatment process to synthesize Zn-Ti oxide nanocomposites. The nanostructures were well characterized and their photocatalyst application was also systematically studied. However, the designed nanostructures are not novel and do not show any better properties. The application in photocatalysis is somewhat a mere combination of old knowledge, but progress from the pre-existing systems is not so sufficient. I have not been completely convinced to recommend its acceptance in Catalysts. There are other specific points that the authors should address before attempting to resubmit this work to a more appropriate journal.

1. Compared with other synthesis strategies, what are the advantages of the plasma jet method for preparing bi-metal oxide nano-composites?

⇒ As we pointed out introduction part, the main advantages of the plasma jet method for preparing metal oxide nanomaterials (MONMs) are described as followings and added more details in the revised manuscript with your comment.

The low temperature, non-equilibrium plasmas provide many advantages for synthesizing size and shape specific metal oxide nanomaterials at even room temperature. Also, if we use the atmospheric plasma, we can obtain the large amounts of MONMs with inexpensive and short time, indicating many advantages for commercially industrial application. Plasma generation equipment has also many advantages of low energy consumption, easy installation cost, atmospheric pressure discharge, and can also be used in various industrial fields.

If you want to get more detailed information, pls. refer to reference 15 as well as a book chapter, entitled “Nanomaterials and their Biomedical Applications”, Springer Series in Biomaterials Science and Engineering 16, 2021, Chapter 2.

2. The authors should provide evidence for the element distribution of Zn and Ti.

⇒ Thank you very much for your indication. Very unfortunately, it is impossible for us to measure EDS element mapping within 10 days. That’s why we can’t give you the evidence for the exact element distribution of Zn and Ti. However, as shown in the Figure 1 (f)~(j) as well as Table 1, we measured EDS spectrum (especially Zn, T, and O) of the five samples and got atomic percentages for the elements of Zn, Ti, O. Even though we do not know whether Ti atoms mixed with Zn atoms homogeneously or not, we obtained atomic mixture ratio. This means that it can be possible of discussing the relative photocatalytic activity since the photocatalytic reaction reflects an average behaviour of catalyst surface. Thus, pls. consider this thing and understand our situation.

3. The kinetics curves in figure 5 are clearly non-linear. Thus, it is not correct to fit the first-order kinetics. Would the reaction constants be calculated within shorter reaction times?

⇒ Thank you very much for your good comment and suggestion. Based on suggestion, we re-plotted the kinetic curves in figure 5(b) and re-calculated the reaction constants within shorter reaction times. The new results are shown in the revised manuscript and appeared the better linearity, but still got out of a straight line. Anyhow, for the better understanding by the readers, we input some sentences in the revised manuscript as followings.

For example, since the concentrations of radicals such as O₂^-· (1D) and OH· are also closely related to the kinetics of dye degradation, it is not exactly fit to the first-order kinetics. This means that more detailed kinetic study including detection of key radicals such as O₂^-· (1D) and OH· radicals is highly desirable to clarify our amazing result. It is well known that the average lifetime of OH· radical (τOH·) in an ambient atmospheric condition is around 0.01∼1 s [32], which is affected by the concentration of reactive gas components such as ozone, VOCs, and NOx. Conclusively, it is very important to note that a photocatalytic reaction is not simple and can’t in general be inferred from the chemical equation for the reaction. That’s why we need a further study to clarify detailed reaction mechanism systematically.

Reviewer 2 Report

Zinc-titanium (Zn-Ti) oxide nanostructures were synthesized using an atmospheric plasma jet and heat treatment process. Photocatalyst application experiments using the the samples were performed. The highest photocatalytic efficiency was obtained of Zn/Ti (2/1). The parameters of governing rate of catalyis were studied and discussed. Thus, we can accept it in the present form.

Author Response

Dear Reviewers,

Thank you very much for refereeing our paper and for giving us your kind corrections and suggestions as well as valuable comments. We have been accepted your fruitful comments, suggestions, and recommendations with all our heart and made our best to revise the manuscript (marked with red color for the corrected parts) with your advice. Thus, hereby, we prepared the answers for you as followings.

Reviewer 2

Zinc-titanium (Zn-Ti) oxide nanostructures were synthesized using an atmospheric plasma jet and heat treatment process. Photocatalyst application experiments using the the samples were performed. The highest photocatalytic efficiency was obtained of Zn/Ti (2/1). The parameters of governing rate of catalyis were studied and discussed. Thus, we can accept it in the present form.

⇒ Thank you very much for refereeing our paper as well as your kind considerations.

Reviewer 3 Report

In my view this may be considered for publication after the following modifications:

     1       Raman  and XRD spectrum of their  used catalysts should be add in manuscript.

2       It is better add grain size of samples from Fig.2 in manuscript.

3      It is better  add elemental map of samples in manuscript.

4      Band gap  of samples should be add in manuscript.

5      No discussion on mechanism, what type of reaction involved and how catalyst work on degradation process of MB. 

            6      The Conclusions section should include:
              * A highlight of your hypothesis, new concepts and innovations.
              * A summary of key improvements compared to findings in literature [give references   

                     to   indicate   key improvements].
              * Your vision for future work

7  The comparison of the other similar research with this work in  a table

Author Response

Dear Reviewers,

Thank you very much for refereeing our paper and for giving us your kind corrections and suggestions as well as valuable comments. We have been accepted your fruitful comments, suggestions, and recommendations with all our heart and made our best to revise the manuscript (marked with red color for the corrected parts) with your advice. Thus, hereby, we prepared the answers for you as followings.

Reviewer 3

In my view this may be considered for publication after the following modifications:

1. Raman and XRD spectrum of their used catalysts should be add in manuscript.

⇒ Based on your suggestion, we added the Raman and XRD spectrum of their used catalysts in the revised manuscript even though some parts were already described in the original manuscript. Thus, pls. check it again from both the original and the revised manuscripts.

2. It is better add grain size of samples from Fig. 2 in manuscript.

⇒ Thank you very much for your good suggestion. Based on our XRD data, we calculated the grain sizes of the samples by utilizing the Scherrer equation. Then, we added the values in the revised manuscript as well as some comparative analysis with BET surface area and SEM particle size. Thus, pls. check it.

3. It is better add elemental map of samples in manuscript.

⇒ Thank you very much for your good indication. Very unfortunately, however, it is impossible for us to measure EDS element mapping within 10 days. That’s why we can’t add the results of element mapping data.

One noticeable thing is that as shown in the Figure 1 (f)~(j) as well as Table 1, we measured EDS spectrum (especially Zn, T, and O) of the five samples and got atomic percentages for the elements of Zn, Ti, O. Even though we do not know whether Ti atoms mixed with Zn atoms homogeneously or not, we obtained atomic mixture ratio. This means that it can be possible of discussing the relative photocatalytic activity since the photocatalytic reaction reflects an average behaviour of catalyst surface. Thus, pls. consider this thing and understand our situation.

4. Band gap of samples should be add in manuscript.

⇒ We had difficulty of getting the exact energy band gaps of our samples due to both extremely low intensity of PL spectra and low optical absorbance or reflectance. That’s why instead of our real data, we added the reference data (see Ref. 27-29) of band gaps for TiO₂, ZnO, ZnTiO₃ crystals in the revised manuscript as followings. Thus, pls. check it up and understand our situation.

The energy band gaps of TiO₂ are 3.0 eV (rutile phase) and 3.2 eV (anatase phase), while the ZnO and ZnTiO₃ crystals have the energy band gaps such as 3.29 eV (wurzite hexagonal ZnO), 3.10 eV (blende cubic ZnO), and 3.06 eV (ZnTiO₃), respectively [27-29]. This means that in the case of Ti-Zn oxide nanocomposites, we can control the energy band gaps between 3.0 and 3.3 eV.

5. No discussion on mechanism, what type of reaction involved and how catalyst work on degradation process of MB. 

⇒ Since it was well known the first-order kinetics for the photocatalytic reaction, we measured the changes of UV-Vis absorption (i.e. п-п* transition). This means that the MB dye has double bonds that would be broken on the catalyst surface by absorbing UV light, resulting in degradation. Thus, the type of reaction is A(MB dye) ↔ P(degraded product). The role of catalyst is allowed a preferred adsorption site as well as lowering the activation barrier and speed-up the reaction. For the better understanding by the readers, we input some details of reaction mechanism, type of reaction, and role of catalyst in the revised manuscript. Thus, pls. check it again.

6.  The Conclusions section should include:
   * A highlight of your hypothesis, new concepts and innovations.
   * A summary of key improvements compared to findings in literature [give references 

to indicate   key improvements].
              * Your vision for future work

⇒ Thank you very much for your suggestions. We revised the conclusion part based on your suggestions. Thus, pls. check it form the revised manuscript.

7. The comparison of the other similar research with this work in a table.

⇒ Thank you very much for your suggestion. With your suggestion, we added the following Table in the revised manuscript. Also, some sentences for the comparison are added as well. Thus, pls. check it from the revised manuscript.

Table 2. Comparison of the other similar research with this work.

Round 2

Reviewer 1 Report

The authors addressed the questions in the revised manuscript. I agree to publish this work if some tiny grammatical issues are solved.

Reviewer 3 Report

Accept in present form