Effect of Ball Milling Speeds on the Phase Formation and Optical Properties of α-ZnMoO₄ and ß-ZnMoO₄ Nanoparticles

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors obtained ZnMoO4  by mechanochemical treatment with two milling speeds applied at 500 and 850 rpm, which shortened the prapration time contrast to other milling method. However, as stated, "Increasing the temperature up to 600 oC did not result in the completion of the chemical reaction, a small amount of MoO3 remained. "I wonder why the experiment under higher tempearature was not conducted. If the temparature futher increased, what would be the results? Besides, many references cited were very long ago, please cite new literatures.

Comments on the Quality of English Language

The English language is basically ok.

Author Response

The authors obtained ZnMoO4 by mechanochemical treatment with two milling speeds applied at 500 and 850 rpm, which shortened the prapration time contrast to other milling method. However, as stated, "Increasing the temperature up to 600 oC did not result in the completion of the chemical reaction, a small amount of MoO3 remained. "I wonder why the experiment under higher tempearature was not conducted. If the temparature futher increased, what would be the results? Besides, many references cited were very long ago, please cite new literatures.

 

Answer: The heat-treatment of the mechanochemically activated sample after 10h at 500rpm is finished at 600 oC, because between 600 – 800 oC MoO3 sublimates and can form dimers (Mo2O6 ), trimers (Mo3O9), and tetramers (Mo4O12) which finally transform into monomers and dimers (A. S. Medvedev and N. V. Malochkina, Russian Journal of Non-Ferrous Metals, 2007, Vol. 48, No. 2, pp. 114–117 ). The melting point of MoO3 is 795 oC. On the other hand, the process of sublimation and melting of MoO3 at low temperature can lead to the formation of non-stoichiometric ZnMoO4 phase or undesirable phases.

We added new literature in the reference list under number 6, 15 and 20.

Reviewer 2 Report

Comments and Suggestions for Authors

The results are interesting, and the manuscript is well-written, with the author presenting the findings clearly. In my opinion, the manuscript merits publication. However, a few points should be further elaborated before final acceptance.

1. The manuscript states, "Up to now, there is only one article on the mechanochemical synthesis for the preparation of ZnMoO4," but it does not elaborate on how this work advances beyond that study. What new insights or improvements does this work bring?
2. What are the challenges associated with the mechanochemical synthesis of ZnMoO4?
3. The authors state that the results demonstrated that the obtained α and β-ZnMoO4 phases possess a new photoluminescence behavior. This statement should be explained more clearly.
4. The manuscript refers to ZnMoO4 phases as "triclinic α-ZnMoO4" and "monoclinic β-ZnMoO4," but later mentions "trigonal α-ZnMoO4." The authors should clarify these phase descriptions to avoid confusion.
5. In the Results and Discussion section, it is stated that after heat treatment at 600°C, "a small amount of MoO3 remained." Does this indicate that the reaction did not reach full completion?
6. The manuscript states that "the average crystallite size is determined at the 30.70° line." The authors should provide a clearer explanation of how the crystallite size was determined.
7. The manuscript states that the band gaps were calculated using Tauc’s equation, assuming ZnMoO4 is a direct bandgap semiconductor (n = 0.5). Can ZnMoO4 also exhibit indirect transitions?

Author Response

Reviewer 2

1. The manuscript states, "Up to now, there is only one article on the mechanochemical synthesis for the preparation of ZnMoO4," but it does not elaborate on how this work advances beyond that study. What new insights or improvements does this work bring?

Answer: In the cited article under number 30, the authors applied for the preparation of metastable ZnMoO₄ mechanochemical treatment with low milling speed (500 rpm). The synthesis was carried out in Pullverisette-6. The synthesis was achieved after 8 years. The authors did not monitor the phase formation, they gave only one X-ray diffraction pattern of the final product. In our article we present detailed information about the phase transformation of initial reagents (MoO₃ and ZnO) depending on the different milling speeds (500 and 850 rpm). In this case the formation of metastable ZnMoO₄ starts after 3h milling time at 850 rpm and finishes after 5 h milling time. We demonstrate the positive effect of the mechanochemical treatment for the preparation of this phase for short reaction time. The X-ray analysis clearly shows the stage of formation of metastable ZnMoO₄ in part 3.1. Phase formation α- and β- ZnMoO₄ nanoparticles

1. What are the challenges associated with the mechanochemical synthesis of ZnMoO4?

Answer: The purpose of this investigation is to check the possibility of the preparation of ZnMoO₄ by simple mechanochemical treatment using pure oxides as reagents, without solvents, complicated operations or sintering of the final product. We established that milling speed led to partial amorphization of the MoO₃ and structural deformation of ZnO. It is predicted to obtain the ZnMoO₄ at a lower temperature than needed for the solid-state reaction. In our case the temperature of synthesis of α-ZnMoO₄ is lower with 100 ^oC as compared to those reported in the literature (S.K. Tiwari et al., J. Mater. Sci. Electron., 2021, 32, 1220081–12889, B. Singh et al., Ceram. Inter. 49, 2023, 38047-38057, Y. Hizhnyi et al., J. Lum. 211, 2019, 127–137). Appling higher milling speed at 850 rpm causes direct synthesis of β-ZnMoO₄ for short reaction time without additional heat treatment. Usually this phase was obtained by hydrothermal synthesis (L.S. Cavalcante, et al, Polyhedron, 2013, 54, 13-25, L.S. Cavalcante, et al ., Colloids Surf. A: Physicochem. Eng. Asp., 2012, 396, 346–351. Y. Keereeta, et al., Superlattices Microstruct., 2014, 69, 253-264, L.Lv et al., J. Nanosci. Nanotechn., 2011, 1, 9506–9512.) We suggested a short method of preparation of the metastable phase β-ZnMoO₄.

1. The authors state that the results demonstrated that the obtained α and β-ZnMoO4 phases possess a new photoluminescence behavior. This statement should be explained more clearly

Answer: We are saying that the α and β-ZnMoO₄ possess new photoluminescence behavior having in mind the literature data. Generally, the emission of α- ZnMoO₄ phase was observed above 500nm (single peak) and below 400 nm (as single and two peaks) which was reported by (Y. Liang et al. Crystal Growth Desing, 2012, Y.Hizhnyi et al.,  J. Lum., 2111, 2019, 127-137, P. Yadar et al., JAC 795, 2019, 446-452-540, Lovisa et al., JAC, 750, 2018, 55, Y. Keereeta et al. Mater. Letters 68, 2012, 265-268, Superlattices Microstruct ,69, 2014, 253-264). On other hand, β-ZnMoO₄ shows broad profile of PL curve with higher maximum above 560nm (R. Firmansyah, at al., Inorg. Chem. Commun., 2023, L.S. Cavalcante et al, Colliods Surf. Eng. Aspects 395, 2012, 346-351, Polyhedra 54, 2013, 13-25) and blue emission at 366nm, at 430/440 nm and blue-green range from 420 nm to 480 nm, (Keereeta et al. Superlattices and Microstructures69, 2014, 253-264, Mafa et al., J. Phys. Chem. C., 2019, 123, 20605 -20616, Wang  et al., Eur. J. Inorg. Chem. 2017, 42, 4939-4946). We observed new emission lines at 590, 615 and 650 nm for α and a blue emission at 403 for β- ZnMoO₄, respectively. In the literature there is no reported data for these emissions. In the revised version of the article, the necessary explanations are given in yellow.

1. The manuscript refers to ZnMoO4 phases as "triclinic α-ZnMoO4" and "monoclinic β-ZnMoO4," but later mentions "trigonal α-ZnMoO4." The authors should clarify these phase descriptions to avoid confusion.

Answer: We agree with this remark and in the revised version of the article, we`ve corrected this error.

1. In the Results and Discussion section, it is stated that after heat treatment at 600°C, "a small amount of MoO3 remained." Does this indicate that the reaction did not reach full completion?

Answer: Yes, the presence of small amount of MoO₃ means that pure single phase of α-ZnMoO₄ was not obtained after heat-treatment. In the part Phase formation of α and β ZnMoO4 nanoparticles, we point out this fact. Increasing the temperature up to 600 ^oC did not lead to the competition of the chemical reaction, small amount of MoO₃ still remained.

1. The manuscript states that "the average crystallite size is determined at the 30.70° line." The authors should provide a clearer explanation of how the crystallite size was determined.

Answer: The average crystallite size (D) resulting from X-ray diffraction broadening was determined using the Scherrer formula at a peak 2θ at 30.70 and it is 35 nm. This explanation was added in the revised article.

1. The manuscript states that the band gaps were calculated using Tauc’s equation, assuming ZnMoO4 is a direct band gap semiconductor (n = 0.5). Can ZnMoO4 also exhibit indirect transitions?

Answer: According to S. Garg et al., J. Alloys Compds., 1010, 2025, 177515, the nature of band gap of ZnMoO₄ can change to indirect under compression (under higher pressure). Most of the articles present values of optical band gap of ZnMoO₄ as direct semiconductors (Cavalcante, L.S.;Polyhedron 54, 2013, 13-25, P. Yadav et al., J. Alloys Compnd, 795, 2019, 446-452,   L.X. Lovisa et al., J. Alloys Compnd, 75o, 2018, 55-70 )

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents an interesting study on the phase formation and optical properties of α-ZnMoO4 and ß-ZnMoO4 nanoparticles. The general structure of the text is good, the language is easily understandable. Some comments should be addressed to improve the quality of the paper.

1. Introduction: the merged citations should be avoided (f. e. [1-9]). Some thought should be written for each cited publication, from which the reader could see why they are relevant to the study.
2. Materials and methods: please, give more information about the selection of the process parameters (milling speed, etc.).
3. It is concluded that the formation of α and β - ZnMoO4 is achieved at different conditions. What could be the reason for this phenomenon?
4. Why was the optical behavior of the studied materials analyzed?
5. The conclusion section should be extended to contain more information about the conducted study.

Author Response

Reviewer 3

The manuscript presents an interesting study on the phase formation and optical properties of α-ZnMoO4 and ß-ZnMoO4 nanoparticles. The general structure of the text is good, the language is easily understandable. Some comments should be addressed to improve the quality of the paper.

1. Introduction: the merged citations should be avoided (f. e. [1-9]). Some thought should be written for each cited publication, from which the reader could see why they are relevant to the study.

Answer: The cited articles from 1 to 9 are related to the optical properties of pure or doped ZnMoO₄ phases. In the revised version of the article, we have separated the references from 1 to 9. In the new reference list, the cited articles from 1 to 5 present data about the photoluminescence spectra of pure ZnMoO₄ phases, while the cited articles from 6 to 10 are connected to photoluminescence data for doped ZnMoO₄. The purpose of our article is to investigate the photoluminescence properties of the pure ZnMoO₄. We have paid attention and provided information about these properties of the reported ZnMoO₄ in the introduction under numbers 1 to 5, 8, 12, 24, 25 and 27.

 

2. Materials and methods: please, give more information about the selection of the process parameters (milling speed, etc.).

Answer: The milling parameters are given in this section, please see below:

The treatment of the initial mixture was performed in a planetary ball mill (Fritsch–Premium line–Pulversette No 7) using the milling speeds of 500 and 850 rpm in air atmosphere and the ball to powder weight ratio was 10:1. To reduce the temperature while milling, the operation was conducted in 15-minute intervals, followed by 5-minute breaks, in accordance with our previous research [31-34].

 

1. It is concluded that the formation of α and β - ZnMoO4 is achieved at different conditions. What could be the reason for this phenomenon?

Answer: The reason for this phenomenon is the energy which is introduced in the reaction mixture during the ball milling process. From X-ray diffraction we have established that lower milling speed led to destruction of the crystal structure of both reagents and to amorphization of MoO₃, that is why in this case additional heat-treatment is needed. The higher energy entered at higher milling speed is enough for destruction of the crystal phase at early stages of the activation. The accumulation of defects and fresh surfaces causes the formation of new chemical bonds and new crystal structure.

1. Why was the optical behavior of the studied materials analyzed?

Answer: This work is a continuation of our previous studies on how mechanochemical treatment influences the optical properties of the final products. We expanded the data for optical properties of as-synthesized ZnMoO₄ phases.

 

5. The conclusion section should be extended to contain more information about the conducted study.

Answer: We agree with this remark; in the revised article the conclusion is extended:

This study demonstrated that milling speed is an important parameter for the preparation of ZnMoO₄. The direct synthesis of metastable monoclinic β -ZnMO₄ was achieved after 5h at higher milling speeds of 850 rpm. The lower milling speed of 500 rpm led to the amorphization of MoO₃ only. In this case additional thermal treatment was carried out to prepare the crystal phase. Thermostable triclinic α-ZnMoO₄ was obtained by combination of ball milling at 500 rpm and calcination at 600 ^oC. By IR spectroscopy was confirmed the presence of main MoO₄ and MoO₆ groups of both samples. The calculated optical band gap of triclinic α-ZnMoO₄ is higher than ones for monoclinic β- ZnMoO₄. The photoluminescence emission in the orange-red range was observed for α-ZnMoO4 due to the presence of deformed MoO₄ units. The blue emission of β-ZnMoO4 was registered as a result of the existence of MoO₆ structural groups. As prepared ZnMoO₄ phases are a promising candidate for application as a phosphor with different emission colors.

 

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

No