Graphene Oxide Chemistry Management via the Use of KMnO₄/K₂Cr₂O₇ Oxidizing Agents

Round 1

Reviewer 1 Report

The authors merge two known GO synthesis methods in attempt to arrive at the flexibly shaped ratio of different chemical functionalization groups. This appears to be mostly successful. Structural and electronic characterization of the obtained materials is rather complex but it seems that there is still room for improvement and the discussion gives no consistent picture.
XRD measurements and analysis is too rudimentary whereas the method is one of the most nanostructure sensitive. Sample preparation should be clearly described and the patterns (avoiding holder scattering) shown in wider angular range (up to e.g. 60 deg two theta) to illustrate presence of a common graphite stacking and a shape of the 10 band. Preferable would be data from two geometries- symmetric reflection and symmetric transmission (with scattering vector mostly in-plane) for which 10 band analysis following Warren method [Warren,Phys.Rev.,59(9) (1941),693; Ruland et al.,J.Appl.Crystall.,35 (2002), 624] can give a measure of the in-plane correlation length affected by in-plane imperfections (e.g. vacancies and holes in the layers). The layers stacking can, and typically does show the graphite-like 002 peak that, compared to 002 of the expanded structure (11 deg.) can allow estimating degree of the layers intercalation due to the out-of-layer functionalization. Comparison of this data with the AFM measured steps (0.7-0.9 nm) could be insightful.
On the other hand SAED data should report beam size giving propper insight into the range of the diffraction data collection. If the selected areas correspond to the nearly micron size TEM pictures then, their Fourier Transform (SAED) can be mostly affected by the graphene layers correlation or in-layer vacancies/faults and less by their corrugations. All the discussion avoids a general structure description merging suggestions from the used different characterization methods. E.g. if the AFM platelets thickness is really 0.7-0.9 nm then why the 002 XRD peak of the expanded structure has width close to 1 deg. what interpreted via Scherrer eq. should correspond to ~ 8 nm stack size ? Structural imperfections can only increase the peak width so the real stack size seems to be even larger. In the referee opinion the structure discussion should be expanded and improved.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The article by Shiyanova et al. describes a facile approach to tailor de content of GO oxygenated groups using KMnO4/K2Cr2O7 oxidizing agents at different ratios. The article is well written and the materials well and extensively characterized by different techniques. However, the novelty of the work is questionable. There are many previous studies dealing with the same topic. See for instance  “Synthesis, Composition, and Properties of Partially Oxidized Graphite Oxides Michal Lojka, Boris Lochman, Ondˇrej Jankovský *, Adéla Jiˇríˇcková, Zden ˇek Sofer and David Sedmidubský” Materials 2019, 12, 2367; doi:10.3390/ma12152367

Rosillo-Lopez, Martin & Salzmann, Christoph. (2018). Detailed Investigation into the Preparation of Graphene Oxide by Dichromate Oxidation. ChemistrySelect. 3. 6972-6978. 10.1002/slct.201801594.

Therefore, the novelty of the work should be clearly highlighted

Other previous studies dealing with the topic, specially those published in the journal should be mentioned in the introduction. See for instance:

Nanomaterials 2020, 10, 239; doi:10.3390/nano10020239

Nanomaterials 2020, 10, 2532; doi:10.3390/nano10122532

Besides, more precise examples about the application the synthesized samples should be provided.

Author Response

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Reviewer 3 Report

The manuscript by Shiyanova et al. deals with the production of GO by oxidizing with either KMnO4, K2Cr2O7 or specific ratios, studying the final products regarding their surface chemistry. It is a nice and comprehensive work, with all proper and vital characterization of the materials. It has a very informative introduction and authors seem to be experts in the field. In addition, this study may be very helpful for researchers working with GO since it is vital for GO preparation to be able to produce samples with specific oxygen functional groups, or C/O ratio. I recommend minor revision having the following comments:

XPS fitting for sample MC3 seems highly questionable if not unrealistic having <0.1 C-C while having 60% C=C and a certain degree of oxygen groups as well (thus many sp3 C atoms). You should consider deconvolute the spectrum once again.

I am not sure I can follow your logic steps on explaining the C/O ratio regarding sample MC6. You seem to have more COOH groups (from the spectrum, not a fitting issue) but why? In addition, we clearly observe a trend on the ratio for more oxidised products with more KMnO4 and less K2Cr2O7, hence why for this sample we observe such an anomaly from the trend?

Where is Figure 3?

XRD patterns of samples MC5 and MC6 (being GOs) are problematic. I don’t remember ever preparing a GO sample not displaying this diffraction peak. In addition, your explanation is questionable. Samples have >20% basal-plane oxygen groups according to XPS, in addition, MC3 for example has less than 30%, further techniques reveal that you have comparable size distributions for the samples having alike platelets etc., so why these samples do not form similar stacking structures upon drying?

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The work is solid, well organized and presented. I simply like the paper.

Sound and consistent results  obtained using different characterization techniques support the conclusions well.

Beside only a few typos and slips in terminology, the text is accurate and well structured.

I recommend the work for publication after the authors address a couple of comments:

• there is a typo in the head of the last column of Table 1. Please correct it.
• the information on the JEOL TEM instrument used for microstructure analyses has be corrected to "JEOL JEM-2100F transmission electron microscope..........with the point resolution of 0.19nm". It is not a "point to point resolution", its just a "point resolution".
• Electron diffraction patterns (eDPs) have to be indexed. 
• Presented TEM data, especially  eDPs contain more information than it is discussed in the manuscript. The eDPs demonstrate very clearly the decrease of the structural order of the graphene sheets as a function of the ratio of the oxidizing agents. Prominent arcing of the rings indicate the corruption of the graphene sheets stacking order in the material. The sheets arrange differently (they rotate relative to one another and the angle of rotation is not accidental) to accommodate the structural transformation (oxidation). This corruption of graphene sheets stacking along the c-axis correlates with the increase of the oxidizing agents ratio, which is very important from my point of view, since it supports the conclusions of the work. Moreover, the fact that the rings in the eDPs become more diffuse evidences the reduction of the dimensions of crystalline regions. So, the long-range order in the plane of a sheet also decreases, the sheet steadily transform to nano-domain structure. I suggest to authors to consider including this in the text, or at least consider this during further studies. 
• The sentence "Such a transformation of ED pattern is commonly attributed to the rotation of diffraction spots due to the variations in the slope of the graphene layer within the aperture of the electron beam" is cumbersome and misleading. The "diffraction spots" do not rotate - the graphite/graphene sheets do. And they rotate not only within the aperture - the progressive changes in eDPs as a function of the ratio of the oxidizing agents evidences that.     Also,  the sentence could be understood as if the "rotation" is induced by the e-beam.

Author Response

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Round 2

Reviewer 1 Report

Good work with collecting XRD data! Transmission data can be acquired simply even by using Scotch tape provided the right, XRD amorphous tape is selected.

Reviewer 2 Report

The manuscript has been improved according to my  suggestions

Reviewer 3 Report

Authors have made all vital corrections

Reviewer 4 Report

The improvements on the manuscript are adequate and just. I recommend the publication of the work in the present form.

 

I have one wish on the text:

Line 462: the expression “rise in defectivity, ….” is reeeeeealy unfortunate. Please consider using the  “the increase in the extend of structural disorder of GO sheets along the MC1 - MC6 row is also evidenced by TEM studies. Figure 7 shows the representative low magnification TEM micrographs of ….“ or similar instead.