Indium Oxide Powder Synthesis in a Low-Current Discharge Plasma at Atmospheric Pressure

Round 1

Reviewer 1 Report

In this paper, the experimental results of the indium oxide powder synthesized by a low-current discharge plasma were reported. The article is interesting, but there are still some problems. Some comments were given as follows.

1.    Line 15, 24, 65, etc. Articles should not be written in the first person. Please try to use the third person passive voice.

2.    There are too many keywords. Keywords should be concise and reflect the core research content of the paper. Testing methods such as "XRD", “TEM”, should not be used as keywords.

3.    Line 20, “The discharge current and mean power were 700 mA and 130 W”. However, in line 107 to 109, the current is 670 mA, and the power is 167W and 120W, respectively. Why?

4.    Line 70, “Since the discharge operates in a current range below the threshold for cathode spot formation”, the authors should give specific values of the threshold for cathode point formation.

5.    How to measure the air pressure during the experiments in this study?

6.    In table1, the difference in discharge power used in Ar gas(120 W) and He gas(167 W) may have an effect on the results. The table should show the atomic percentages of In, O and Mo at the same position for the products prepared under the different gases.

7.    Line 320, “(up to 1 A)”, The data in the conclusion should be consistent with the content of the article.

Author Response

Thanks for the valuable comments. The answers are presented here. Corrections in the text of the manuscript are highlighted in red.

1. Line 15, 24, 65, etc. Articles should not be written in the first person. Please try to use the third person passive voice.

Text has been corrected.

2. There are too many keywords. Keywords should be concise and reflect the core research content of the paper. Testing methods such as "XRD", “TEM”, should not be used as keywords.

Text has been corrected.

3. Line 20, “The discharge current and mean power were 700 mA and 130 W”. However, in line 107 to 109, the current is 670 mA, and the power is 167W and 120W, respectively. Why?

This discrepancy has been corrected. The abstract contains the exact values of the parameters of the discharge in argon and helium.

4. Line 70, “Since the discharge operates in a current range below the threshold for cathode spot formation”, the authors should give specific values of the threshold for cathode point formation.

This sentence has been corrected in this way: “The discharge operates in a glow mode. A characteristic feature of this mode is a sufficiently high voltage: more than 100 times higher than that of the cathode arc voltage with an indium cathode [37]. This circumstance in this case eliminates the problem of erosion of the cathode material in the form of microdroplets”.

5. How to measure the air pressure during the experiments in this study?

Direct measurements of the pressure in the ambient air were not carried out. However, information about the change in atmospheric pressure in Tomsk, Russia, where the research was located, was displayed on the online service:

https://barometricpressure.app/results?lat=56.488712&lng=84.952324

For example, the average value of atmospheric pressure in October 2022, when these studies were carried out, was 752.9 ± 6.38 mm Hg.

6. In table1, the difference in discharge power used in Ar gas(120 W) and He gas(167 W) may have an effect on the results. The table should show the atomic percentages of In, O and Mo at the same position for the products prepared under the different gases.

Partially agree. Different discharge voltage values during its operation in argon and helium flows are due to the difference in the physical properties of the atoms of these gases – ionization potentials, ionization cross section, etc. At the specified discharge parameters, the plasma source generated flows of indium atoms most stably. In connection with this experimental fact, it should be concluded that in the present work this was an essential condition for the study. For more detailed studies of the processes of obtaining nanosized indium oxide powders with identical discharge parameters, it is necessary to create a special power source, which is beyond the scope of the tasks set and, most likely, will be the topic of further research.
According to the comment of Table 1, lines were added to it to indicate the conditions for the functioning of the discharge in the process of generating indium oxide powders.

Table 1. TEM/EDS analysis of In₂O₃ powder particles elemental composition.

7. Line 320, “(up to 1 A)”, The data in the conclusion should be consistent with the content of the article.

This discrepancy has been corrected. The conclusion contains the exact value of the discharge current.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments to Author

The author proposed the manuscript entitled “Indium oxide powder synthesis in a low-current discharge plasma at atmospheric pressure”. The manuscript is well written and can be suitable for acceptance, but there are some problems in this work preventing it from further consideration. The following are some points that might be useful to refine the context.

Category: “Major revision”

Comment:

1.      There is a lack of correlation in the introduction section, I strongly recommend the author rewrite the introduction starting with the In₂O₃ material property or background, what other competitive wide bandgap materials have been studied so far, the need for In₂O₃ or its advantages of it over other semiconducting materials, previously used synthesis technique for In₂O₃, economic or another specific advantage of the present plasma technique, and so on.

2.      What is the main novelty of this work, and what are the material's basic physical or electrical properties? I would suggest the author accredit the related In₂O₃ article in the introduction section.

3.      It would be more attractive for the readers if the author modified the eye-catching schematic Figure 1.

4.      Please check the Gas flow rate description is it really “1 l/min.”? usually gas flow rate is described in SCCM standards.

5.      Is it possible for the author to measure or estimate the surface tension of the molten metal?

6.      What is the range of applied current for the JEM 2100 transmission electron microscope, and what are the other specific experimental operating condition for all instruments used?

7.      The XRD reference data card (pdf card #03-065-3170) is the JCPDS reference or any other, please mentioned it clearly.

8.      How are the morphology, crystallinity, and particle size growth affected for the different conditioned samples (Figure 5-7), is there any correlation between these properties and the synthesis condition?

9.      The author must estimate the particle size from the XRD data using the Debye Scherrer formula, and compare the obtained data with the estimated particle size from TEM analysis.

10.  Did the author estimate the (= 0.1013 nm) value, it is identical for all three samples, please provide the formula used for the estimation of a value.

11.  What is the effect of current variation on cathodic thermal erosion, did the author check it?

12.  Figure format, font size, thickness, and used symbol should be consistent throughout the manuscript, it must be in accordance with journal guidelines.

13.  The author should provide the abbreviation at the of the manuscript before the reference section.

14.  English correction is highly needed to improve the manuscript's grammatical errors and typo mistakes.

Comments for author File: Comments.pdf

Author Response

Thanks for the valuable comments. The answers are given here. Corrections in the text of the manuscript are highlighted in red.

Comment:

1. There is a lack of correlation in the introduction section, I strongly recommend the author

rewrite the introduction starting with the In2O3 material property or background, what other

competitive wide bandgap materials have been studied so far, the need for In2O3 or its

advantages of it over other semiconducting materials, previously used synthesis technique for

In2O3, economic or another specific advantage of the present plasma technique, and so on.

2. What is the main novelty of this work, and what are the material's basic physical or

electrical properties? I would suggest the author accredit the related In2O3 article in the

introduction section.

According to the first and second points, the section "Introduction" is supplemented with information in accordance with these comments, including additional references to works related to this study.

3. It would be more attractive for the readers if the author modified the eye-catching

schematic Figure 1.

The figure has been modified.

4. Please check the Gas flow rate description is it really “1 l/min.”? usually gas flow rate is

described in SCCM standards.

Text has been corrected.

5. Is it possible for the author to measure or estimate the surface tension of the molten metal?

In accordance with this comment, the surface tension of molten indium was estimated. A related sentence and a link to the corresponding work, which describes the experimental technique for measuring this parameter, has been added to the text of the manuscript.

The surface tension is 518 mN/m, which corresponds to a surface energy of 5.18 10^-5 J/cm². It is easy to estimate the energy density dissipated near the working surface of the cathode in one pulse of the discharge current. For example, when the discharge operates in an argon flow with a current of 670 mA, at a voltage of 180 V, and a pulse duration of 10 μs, with an active cathode surface area equal to the area of a hemisphere with a radius of 4 mm - 0.25 cm², it is approximately 480 10^-5 J/cm² . Thus, it can be concluded that sufficient energy is imparted to the surface of the meniscus formed by molten indium to overcome the surface tension.

6. What is the range of applied current for the JEM 2100 transmission electron microscope,

and what are the other specific experimental operating condition for all instruments used?

The applied current in TEM does depend on the exposition mode. In particular, for collecting of the bright-field images in TEM the current was the largest ~104.5 μA. To get the high-resolution TEM images, we employed the smaller condenser aperture and the current dropped up to 103.7 μA. Finally, in the nano-beam diffraction mode, the applied current was the smallest ~103.1 μA.

7. The XRD reference data card (pdf card #03-065-3170) is the JCPDS reference or any other,

please mentioned it clearly.

The reference XRD pattern used for indexing of experimental data was taken from the commercially available ICDD (International Centre for Diffraction Data) Database, as mentioned in ref. 29. We described the pdf card number (#03-065-3170) as well as a symmetry group and a lattice constant in the manuscript. There are only two modification of In2O3: cubic and hexagonal. The former was found in our study.

8. How are the morphology, crystallinity, and particle size growth affected for the different

conditioned samples (Figure 5-7), is there any correlation between these properties and the

synthesis condition?

The text of the manuscript contains several sentences describing the effect of synthesis conditions on the morphology and structure of particles.

“Structural studies of the dispersed individual nanosized powder particles performed by transmission electron microscopy and powder aggregation in the form of a coating using X-ray diffractometry show that the data obtained by these inde-pendent methods are in good agreement. It was found that the structure of powder particles corresponds to a single phase of indium oxide (III) with body-centered cubic (bcc) lattice with a lattice parameter a = 1.013 nm. Energy dispersive analysis shows that, within the framework of this work, these particles are represented by non-stoichiometric indium oxide In₂O₃ with a nanocrystalline structure (<d> = 13–16 nm), regardless of the working gas (Ar or He).”

9. The author must estimate the particle size from the XRD data using the Debye Scherrer

formula, and compare the obtained data with the estimated particle size from TEM analysis.

Indeed, the size of the coherent scattering regions may be estimated by XRD using a peak broadening and Debye-Scherrer formula. Taking into account the splitting of Ka-dublet of the single peaks belonging to In₂O₃, the mean particle size calculated by XRD is ~10 nm that is slightly lower than that estimated in TEM (~15 nm). The possible reason of this discrepancy is a presence of defects (dislocations and stacking faults) inside the particles (Fig. 6c), so the perfect (mosaic) crystal packing is violated.

10. Did the author estimate the (= 0.1013 nm) value, it is identical for all three samples, please

provide the formula used for the estimation of a value.

The formula used for the estimation of the lattice parameters is given in Materials and Methods. We made an annoying misprint, because the value of the lattice parameter for In₂O₃ phase was a = 1.013 nm rather than 0.1013 nm. The accurate estimation of the lattice constant was done for symmetric (Bragg-Brentano) scheme of X-ray analysis. Since the larger measurement error of the lattice parameter in asymmetric scheme of X-ray analysis due to elastic stresses, etc., we did not perform such calculations in the paper. For all studied samples, the lattice parameter of In₂O₃ was practically the same and all SAED patterns are well indexing using the value of a = 1.013 nm.

11. What is the effect of current variation on cathodic thermal erosion, did the author check it?

Yes, such an effect exists.

The nature of the operation of a glow discharge at atmospheric pressure in the mode of generating flows of indium atoms for the synthesis of indium oxide nanopowder is such that the dependence of the temperature erosion of the cathode insert on the discharge current has a threshold character. Under the conditions of the present study, the transition to this regime was observed at the flat part of the discharge current pulse at a level of 350 mA. This was accompanied by the appearance in the spectrum of the optical radiation of the discharge plasma of lines of excited indium atoms of low intensity compared to the intensity of the lines of argon or helium. An increase in the discharge current led to an increase in the intensity of the lines of metal atoms, which is equivalent to an increase in their concentration in the total particle flux. This process is described in more detail using the example of obtaining fluxes of magnesium atoms in a discharge at atmospheric pressure in the work J. Appl. Phys. 127, 213303 (2020); https://doi.org/10.1063/5.0006239.

12. Figure format, font size, thickness, and used symbol should be consistent throughout the

manuscript, it must be in accordance with journal guidelines.

Agree. Done.

13. The author should provide the abbreviation at the of the manuscript before the reference

section.

Agree. Done. The explanation of the main abbreviations is given in the abstract and in the introduction.

14. English correction is highly needed to improve the manuscript's grammatical errors and

typo mistakes.

The English correction was performed by Dr. Ian G. Brown (Lawrence Berkeley National Laboratory). He is specialist in the field of gas and vacuum arc discharges physics, material physics. He is originally Australian and is a native speaker.

Author Response File: Author Response.docx