Exfoliation Behavior of Large Anionic Graphite Flakes in Liquid Produced by Salt-Assisted Ball Milling

Round 1

Reviewer 1 Report

The authors report the  pretreated anionic graphite flake can lead to higher efficiency on obtaining the graphene flakes from the liquid-phase exfoliation (LPE)

Overall, the manuscript was well organized. Also, the study was well conducted and the discussion is convincing. However, the graphene scalable production methods have been intensively reported in the past 10 years. And much more efficient methods based on the LPE have been reported, like shear force assisted exfoliation. In particular, electrochemical exfoliation is regarded as even better efficient approach than others.  Thus this work shows lower novelty and less improvement in this regard. Also, there are some contradictions from the scientific parts. Thus this referee recommends the manuscript not to be published in this present form. Other comments ware shown below.

 

The reference review didn't include the more recent work on the scalable production of graphene flakes. Especially a similar concept by ion electrolyte intercalation and exfoliation methods that publish whin these three years.   The AFM step profile shows zig-zag single, it seems more like the noise single rather than the true AFM single. Please check this.  The Raman spectra in Figure8, the non-symetric 2D peak reveal the fact of multilayered graphene falkes(>10 nm). This result is not consistent with the AFM results, the TEM maybe need to verify this layer thickness. And also the statistical analysis on the thickness distribution should be provided. The layer number should be probed by AFM or HRTEM, not XRD The Raman can only tell the degree of crystallinity or the defect density. While the C/O or the amount of oxygen functional groups can be obtained from XPS. Thus the XPS has to be provided to further understand their as-prepared graphene quality.       

Author Response

Please check the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

The work reported by Arao, et al. deals with the ball milling-assisted graphite flake exfoliation. The research is well conducted. The works presents a good quality of grammar and editing. In the Reviewer’s opinion, this manuscript falls within a scope of the Journal. I recommend to accept this manuscript for publication after minor revisions:

References are needed after some sentences in the text, such as page 1 lines 30-31, page 5 lines 113-114, page 14 line 260, and many other. Thermogravimetric analyses (TGA) are recommended to be performed, in order to study the thermal stability of the obtained carbon materials. The use of a word “nanosheet” is not recommended, since graphene-family materials are thin enough to refer them as “sheets”. See recommended nomenclature in the editorial article by Carbon journal, DOI: 10.1016/j.carbon.2013.08.038. Did the Authors consider the influence of viscosity of the solvents (acetone, MEK, IPA) on the exfoliation efficiency (subsection 3.3)? Conclusions: I cannot find any note on the topic of Raman analyses, despite the fact that these experiments showed some very important results on the Id/Ig ratio. Conclusions: please highlight your future plans on this research topic. There are some editing mistakes, such as page 15 line 288, and other.

Author Response

Reviewer 2

The work reported by Arao, et al. deals with the ball milling-assisted graphite flake exfoliation. The research is well conducted. The works presents a good quality of grammar and editing. In the Reviewer’s opinion, this manuscript falls within a scope of the Journal. I recommend to accept this manuscript for publication after minor revisions:

References are needed after some sentences in the text, such as page 1 lines 30-31, page 5 lines 113-114, page 14 line 260, and many other. Thermogravimetric analyses (TGA) are recommended to be performed, in order to study the thermal stability of the obtained carbon materials. The use of a word “nanosheet” is not recommended, since graphene-family materials are thin enough to refer them as “sheets”. See recommended nomenclature in the editorial article by Carbon journal, DOI: 10.1016/j.carbon.2013.08.038. Did the Authors consider the influence of viscosity of the solvents (acetone, MEK, IPA) on the exfoliation efficiency (subsection 3.3)? Conclusions: I cannot find any note on the topic of Raman analyses, despite the fact that these experiments showed some very important results on the Id/Ig ratio. Conclusions: please highlight your future plans on this research topic. There are some editing mistakes, such as page 15 line 288, and other.

 

References are needed after some sentences in the text, such as page 1 lines 30-31, page 5 lines 113-114, page 14 line 260, and many other.

Ans. In the case of the text in page1 lines 30-31, there is no references. The process for growth of high quality of graphite in the ground has not clarified yet.

And also there is no reference for line 112-113. “Any residual water in the graphite powder can inhibit the mechanochemical reaction, so drying is an important step” This is our experimental knowhow.

With respect to line 260, we have added the proper reference based on your advice.

 

Thermogravimetric analyses (TGA) are recommended to be performed, in order to study the thermal stability of the obtained carbon materials.

Ans. It is good idea to conduct TGA. In fact, it was reported that we can determine the size of graphite by TGA results, because thermal stability of graphite is dominated by the edge of graphite.

(Buzaglo et al. Graphite-to-graphene: Total Conversion, Advanced Materials, Vol. 29, 1603528, 2017)

In our case, we have confirmed that there is no significant difference between natural graphite and anionic graphite with respect to TGA results. This is because the flake size of anionic graphite is the same with the natural graphite. In addition, adsorbed salt was in the order of 0.1 wt%, which is difficult to detect by TGA.

In any case, TGA is conventional tool for characterization of graphite or graphene. We would like to make detail discussion for TGA results in the future.

 

The use of a word “nanosheet” is not recommended, since graphene-family materials are thin enough to refer them as “sheets”. See recommended nomenclature in the editorial article by Carbon journal, DOI: 10.1016/j.carbon.2013.08.038.

Ans. Thank you for your kind advice. We have removed a word “nanosheet”. Instead of using nanosheet, we use FLG. In addition, we have modified the figure 4 (a). We also added the paper you told as reference 3.

 

Did the Authors consider the influence of viscosity of the solvents (acetone, MEK, IPA) on the exfoliation efficiency (subsection 3.3)?

Ans. Thank you for your comment. We also feel the viscosity of the solvent also has the dominant effect on exfoliation of graphite. In fact, the recent paper reveals higher viscosity is preferable for exfoliation compared to the solvent with lower viscosity in the case of shear exfoliation.

Diasio et al. “The effect of solvent viscosity on production of few-layer graphene from liquid-phase exfoliation of graphite” MRS Advances Vol. 4, 241-247.

In our case (sonication), the same tendency might be confirmed because the exfoliation degree of acetone, which shows lowest viscosity, was worst. But we feel more systematic experiment will be required to discuss how the viscosity of solvent affect the exfoliation behavior of graphite. Therefore, we didn’t add the explanation about the viscosity.

 

Conclusions: I cannot find any note on the topic of Raman analyses, despite the fact that these experiments showed some very important results on the Id/Ig ratio. Conclusions: please highlight your future plans on this research topic.

Ans. Based on your advice, we have added the text below.

“The defects of graphene obtained from anionic graphite is low (I_D/I_G=0.19-0.22) based on Raman analysis.”

In addition, we have added our future plans for this research.

“We have found that edge fragmentation of anionic graphite during LPE reduces the lateral size of exfoliated graphene. The new technique that avoids the edge fragmentation of anionic graphite will be required to obtain high quality and large size of graphene.”

 

There are some editing mistakes, such as page 15 line 288, and other.

Ans. We’re appreciate your comment. We have checked the text, and modified the editing mistakes.

 

 

Reviewer 3 Report

Dear Sir,

The article entitled "Exfoliation behavior of large anionic graphite flake 2 in liquid produced by salt-assisted ball milling” by the authors Y. Arao, et al is quite interesting for the scientific community especially for researchers involved in the use of graphene flakes.

In the article the authors synthesized by salt-assisted ball milling an Anionic graphene that has the same size  and layered structure as natural graphite. So I strongly recommend it to be published.

Author Response

Thank you for reviewing our paper.

Reviewer 4 Report

The authors present a systematic study of exfoliation of anionic graphite flakes in liquid. They achieve a significantly higher yield of graphene as compared with exfoliation of natural graphite flakes under similar conditions. The presentation is clear, convincing, and easy to follow. The ultra-high exfoliation efficiency of anionic graphene powders is of technological interest. The proposed mechanism of edge repulsion inspires further theoretical and experimental studies. I recommend its publication in Processes. 

Author Response

Thank you for reviewing our paper.

Reviewer 5 Report

1 - The sentences “The “top-down” approach to producing graphene (i.e., from graphite to graphene) is far more cost effective compared to the “bottom-up” approach (i.e., assembling graphene from a substrate), because natural graphite is cheap (1-10 USD/kg) and abundant. Exfoliation of this highly-crystalline graphite yields graphene nanoplatelets (more than 10 layers), few-layer graphene (FLG, 2 to 5 layers), or mono layer graphene—depending on the exfoliation degree of graphite.”, in the Introduction section, needs references. Moreover, it is not clear what the Authors want to say with “assembling graphene from a substrate”.

2 – The Authors should indicate the error associated with the average diameter of graphite, in the sentence “The average diameter of natural graphite and anionic graphite used for LPE process are 13 m and 11.2 m, respectively.” Also, in the sentence “Uniform thickness of 0.8 nm and a lateral size of 50-400 nm was…”

3 – Figure 11 should be changed to Figure 10.

4 – The sentence “This is attributed to fragmented of the graphite flakes during the LPE process, causing size reduction and thus a gradual increase in the measured graphene concentration.” should be supported by SEM images or AFM analysis.

5 – “We performed elemental analysis for exfoliated powder and sediment powder by 318 EDS, and found that the powders collected from dispersion had higher potassium content (0.18 319 wt%) compared to the sediment powder (0.02 wt%)”. Which is the limit detection of the equipment?

Author Response

Reviewer 5

 

1 - The sentences “The “top-down” approach to producing graphene (i.e., from graphite to graphene) is far more cost effective compared to the “bottom-up” approach (i.e., assembling graphene from a substrate), because natural graphite is cheap (1-10 USD/kg) and abundant. Exfoliation of this highly-crystalline graphite yields graphene nanoplatelets (more than 10 layers), few-layer graphene (FLG, 2 to 5 layers), or mono layer graphene—depending on the exfoliation degree of graphite.”, in the Introduction section, needs references. Moreover, it is not clear what the Authors want to say with “assembling graphene from a substrate”.

Ans. Thank you for your kind comment. We added the proper reference that shows the definition of graphene. “Assembling graphene from substrate” is difficult to understand for reader. Therefore, we modified the sentence as “chemical vapor deposition method”.

 

2 – The Authors should indicate the error associated with the average diameter of graphite, in the sentence “The average diameter of natural graphite and anionic graphite used for LPE process are 13 m and 11.2 m, respectively.” Also, in the sentence “Uniform thickness of 0.8 nm and a lateral size of 50-400 nm was…”

Ans. Error means standard deviation or data scattering? We have provided the data in the supplemental materials (different file.) We believe that readers can understand the distribution of flake size from Fig. S1 and Fig. S3.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3 – Figure 11 should be changed to Figure 10.

Ans. Thank you for your advice. It was our mistake. We have checked the text carefully, and modified it.

 

4 – The sentence “This is attributed to fragmented of the graphite flakes during the LPE process, causing size reduction and thus a gradual increase in the measured graphene concentration.” should be supported by SEM images or AFM analysis.

Ans. The size reduction of graphite occurs during sonication, and small graphite flake easily exfoliated to few-layer graphene. This phenomenon was reported by Barwich et al.

“Barwich, S.; Khan, U.; Coleman, J. N., A Technique To Pretreat Graphite Which Allows the Rapid Dispersion of Defect-Free Graphene in Solvents at High Concentration. The Journal of Physical Chemistry C 2013, 117, (37), 19212-19218.”

Therefore, we have added the above reference.

 

 

5 – “We performed elemental analysis for exfoliated powder and sediment powder by 318 EDS, and found that the powders collected from dispersion had higher potassium content (0.18 319 wt%) compared to the sediment powder (0.02 wt%)”. Which is the limit detection of the equipment?

Ans. The detection accuracy of EDS is approximately 0.01 wt%.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Although the author modifies and answers this referee's question. There still some issue need to clarify. 

The electrochemical exfoliation method for obtaining graphene can control the surface functionality by controlling the recipe such as voltage bis and electrolyte. And it can well be dispersed in water-based solution and been applied as PU/graphene composite(such as but not limited to this reference: Nanoscale, 10, 12612 - 12624 (2018)). So this argument actually not exactly correct.  Moreover, if the graphene by this method can spontaneously be dispersed in water, what kind of group on it to make the surface convert to hydrophobic?  This may need to detail.      This referee believes that the statistical analysis on layer number should be provided. This is crucial information to detail the as-prepared graphene is single or few-layered graphene. It's not easy to got the 100% single-layer graphene by this method.    

Author Response

Please check the attached file.

Author Response File: Author Response.docx

Reviewer 5 Report

Since the potassium content is too low to be considered if the EDS precision is not mentioned, I suggest adding EDS equipment detection accuracy to the "Characterization" sub-section.

Author Response

 

Reviewer 5

Q.1 Since the potassium content is too low to be considered if the EDS precision is not mentioned, I suggest adding EDS equipment detection accuracy to the "Characterization" sub-section

 

A.1 We’re appreciate your comment. We have discussed with the specialist for EDS equipment, and the accuracy of the element was approximately 0.03 at%. We have added the detection accuracy of the equipment in “Characterization” section. (line 151).

“The accuracy of the elemental analysis was 0.03 at% in our experimental condition.”