From Behavior of Water on Hydrophobic Graphene Surfaces to Ultra-Confinement of Water in Carbon Nanotubes

Round 1

Reviewer 1 Report

The authors present and interesting first-principles study of water/graphene and water/carbon-nanotube reactions. The study is based on density functional theory and ab initio molecular dynamics simulations. The results presented indicate strong affinity of graphene carbon atoms for protons. Protons themselves functionality the otherwise inert surface of graphene and allow attachment of oxygen, with formation of OH molecules. In my opinion, the ms could be published after mandatory revision. I provide some suggestions to improve the text. I also have some comments which the authors may want to keep in mind for future investigations.

 

• I do not see mentioned in the text the use of van der Waals corrections. These are surely important to model adsorbate/graphene interactions. Why were these neglected?
• The correct name is “density functional theory” (DFT), not “functional density theory”. Please correct in the abstract and throughout the text.
• Some claims are too strong. For example, in the abstract, “our calculations demonstrate …”. DFT based on semi-local exchange and correlation approximations is known to be not particularly accurate for modelling chemical reactions [1] (which is the case here when deprotonation of water occurs), nor van-der-Waals-like adsorption of adsorbates on surfaces [2]. The authors should inform readers of DFT limitations (see my next comment) and change statements like “results demonstrate” into “results indicate” or “results suggest”.
• Related to my previous comment, the authors should indicate in the text something like “despite known DFT limitations in describing molecular adsorption or chemical reactions [1, 2], DFT-based ab initio molecular dynamics is currently the most accurate and reliable tool for revealing gas/surface reaction pathways [3] and adsorbate diffusion [4].”
• The surface cell used in this work is relatively small (32 atoms). Did the authors make sure that the water (or other molecules) self-interactions due to periodic-boundary conditions are negligible? How is this issue treated in the “effective screening medium” approach ?
• In the methods section, I see both Rydberg and Hartree units. Later in the text, eV are used. Can the authors please use only one energy unit? Also, for the self-consistent energy, there is a convergence criterion of 10^-6 Hartree. Is this Hartree/atom or Hartree/supercell? Please specify.
• This is a comment which may serve the authors in future investigations. The 2D nature of graphene makes it a strongly anharmonic surface, with relatively broad out-of-plane vibrations in comparison to interatomic C-C bond lengths [5, 6]. A 32-atom cell can accommodate very few surface-phonon wavelengths. Proper description of the phonon wavelength degrees of freedom may be crucial, especially at high temperatures.
• That graphene has high affinity for protons is not particularly new. See, for example, ref. [7]. The authors should provide this (and possibly a few other) references to give credit to previous findings for proton adsorption on graphene.

 

 

• [1] A.J. Cohen, P. Mori-Sanchez, W.T. Yang, Insights into current limitations of density functional theory, Science 321 (2008) 792.
• [2] L. Schimka, J. Harl, A. Stroppa, A. Gruneis, M. Marsman, F. Mittendorfer, G. Kresse, Accurate surface and adsorption energies from many-body perturbation theory, Nature Materials 9 (2010) 741.
• [3] D.G. Sangiovanni et al., Ab initio molecular dynamics of atomic-scale surface reactions: insights into metal organic chemical vapor deposition of AlN on graphene, Physical Chemistry Chemical Physics 20 (2018) 17751.
• [4] A. Jamnig et al., Atomic-scale diffusion rates during growth of thin metal films on weakly-interacting substrates, Scientific Reports 9 (2019) 6640.
• [5] A. Fasolino, J.H. Los, M.I. Katsnelson, Intrinsic ripples in graphene, Nature Materials 6 (2007) 858.
• [6] K.V. Zakharchenko et al., Finite Temperature Lattice Properties of Graphene beyond the Quasiharmonic Approximation, Physical Review Letters 102 (2009) 046808.
• [7] J. Son et al., Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor Nature Communications 7, 13261 (2016)

Author Response

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

English language and style

( ) Extensive editing of English language and style required
( ) Moderate English changes required
(x) English language and style are fine/minor spell check required
( ) I don't feel qualified to judge about the English language and style

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	( )	(x)	( )	( )
Is the research design appropriate?	( )	(x)	( )	( )
Are the methods adequately described?	( )	(x)	( )	( )
Are the results clearly presented?	(x)	( )	( )	( )
Are the conclusions supported by the results?	(x)	( )	( )	( )

Comments and Suggestions for Authors

The authors present and interesting first-principles study of water/graphene and water/carbon-nanotube reactions. The study is based on density functional theory and ab initio molecular dynamics simulations. The results presented indicate strong affinity of graphene carbon atoms for protons. Protons themselves functionality the otherwise inert surface of graphene and allow attachment of oxygen, with formation of OH molecules. In my opinion, the ms could be published after mandatory revision. I provide some suggestions to improve the text. I also have some comments which the authors may want to keep in mind for future investigations.

 

• I do not see mentioned in the text the use of van der Waals corrections. These are surely important to model adsorbate/graphene interactions. Why were these neglected?

Author reply: We are aware of DFT lacking and of DFT-D performances. Indeed, the DFT-D2 method is numerically extremely efficient and, in test calculations, produces accurate binding energies, especially if combined with some of the double hybrid functionals compared to GGA. Unfortunately, some drawbacks can be enumerated. Foremost, it lacks flexibility and dependence on the electron distribution. And so, while polarizability of molecules depends on their electronic states, the C6 coefficients assigned to each atom are rigid. Ultimately, the method does not distinguish between di- and trivalent atoms. However, this final issue was directed in the DFT-D3, TS and BJ methods. The DFT-D3 method takes into account the chemical environment of each atom by assigning a coordination number. In TS, the C6 coefficients are rescaled depending on the charge distribution, while, in BJ, they are calculated on the basis of the exchange-hole model, which makes this method completely non-empirical. Although these adjustments improve qualities of these methods, it has never been proven convincingly that any of these approaches work accurately for solids. Ultimately, the operation of all these methods depends strongly on a choice of empirical parameters. A recent sensitivity analysis has shown that the main source of uncertainty in these calculations is sealed in the damping function. This solution means that the adjustment of C6 coefficients performed on-the-fly in the TS and BJ method is relatively insignificant, and one should focus on adjusting the van der Waals radii instead, as is it is done in DFT-D3. As a result, the shift in energy obtained using more advanced methods will be always submitted to discussion (see “van der Waals Interactions in Density-Functional Theory: Implementation and Applications”, Doctoral dissertation of A. Gulans ISBN 978-952-60-4472-9). 

We add a sentence about this choice in the method. “Note that the van der Waals corrections were not taken into account in our calculations. The choice of empirical parameters dedicated to the modeling of these corrections in DFT could increase the main source of uncertainty in our calculation. This would lead to shifts in energy which will always be subject to discussion” (Ref “van der Waals Interactions in Density-Functional Theory: Implementation and Applications”, Doctoral dissertation of A. Gulans ISBN 978-952-60-4472-9”). Line 115-119.

 

• The correct name is “density functional theory” (DFT), not “functional density theory”. Please correct in the abstract and throughout the text

Author reply: we thank the Referee for his careful reading. We have corrected these mistakes.

 

 

• Some claims are too strong. For example, in the abstract, “our calculations demonstrate …”. DFT based on semi-local exchange and correlation approximations is known to be not particularly accurate for modelling chemical reactions [1] (which is the case here when deprotonation of water occurs), nor van-der-Waals-like adsorption of adsorbates on surfaces [2]. The authors should inform readers of DFT limitations (see my next comment) and change statements like “results demonstrate” into “results indicate” or “results suggest”.

Author reply: According to Referee remark, we have modified the sentences where our claims were too strong. For instance, “According to our calculations” replaces “our calculations demonstrate” (line 15 in the abstract) and “From all of our studies, we have shown here a very strong affinity of the carbon wall,…” replaces “From all of our studies, we demonstrated…”  (line 409 in the conclusion).

 

• Related to my previous comment, the authors should indicate in the text something like “despite known DFT limitations in describing molecular adsorption or chemical reactions [1, 2], DFT-based ab initio molecular dynamics is currently the most accurate and reliable tool for revealing gas/surface reaction pathways [3] and adsorbate diffusion [4].”

Author reply: According to Referee remark we have added a small discussion about the limitations of the DFT and the References dedicated to. “Although it is most frequently used in the description of the electronic structure of a system, the DFT based on the generalized gradient approximation has certain limitations, in particular for the modeling of chemical reactions [1] and the estimation of gas-phase energy barriers [ref-1]. DFT-GGA may also not perform well for many molecule-metal surface reactions nor van-der-Waals-like adsorption on surfaces [2]. Ab-initio molecular dynamics based on density functional theory are more reliable and accurate in describing molecule−surface interaction, reaction pathways [3], adsorbate diffusion [4] and energy exchange as it permits surface-atom movement and also includes the temperature effect [ref-2]”. Line 100-108

• The surface cell used in this work is relatively small (32 atoms). Did the authors make sure that the water (or other molecules) self-interactions due to periodic-boundary conditions are negligible? How is this issue treated in the “effective screening medium” approach ?

Author reply: Although the surface cell is relatively small, the ESM method allows to restrict liquid molecules within a given region by introducing a cubic barrier potential of a given value. It is called the “ESM wall” and its position can be calibrated in the input file, as the distance between the upper edge of the cell and the origin of the potential barrier. This potential wall allows to avoid the overlap of periodic images of water molecules among others.

 

• In the methods section, I see both Rydberg and Hartree units. Later in the text, eV are used. Can the authors please use only one energy unit? Also, for the self-consistent energy, there is a convergence criterion of 10^-6 Hartree. Is this Hartree/atom or Hartree/supercell? Please specify.

Author reply: For self-consistent energy, the convergence criterion is given for the supercell (Hartree/supercell). We have specified it. We decide to let the primary energy unit used in the code in order to be consistent with the unit adopted in the open-MX community. However, we indicated also the energies in eV to uniform the units.

• This is a comment which may serve the authors in future investigations. The 2D nature of graphene makes it a strongly anharmonic surface, with relatively broad out-of-plane vibrations in comparison to interatomic C-C bond lengths [5, 6]. A 32-atom cell can accommodate very few surface-phonon wavelengths. Proper description of the phonon wavelength degrees of freedom may be crucial, especially at high temperatures.

Author reply: we thank the Referee for this interesting comment. The role of the vibrations on the functionalization of the carbon surface will be an amazing challenge.

 

• That graphene has high affinity for protons is not particularly new. See, for example, ref. [7]. The authors should provide this (and possibly a few other) references to give credit to previous findings for proton adsorption on graphene.

 Author reply: Recent references have been added to underline the high affinity of protons for carbon. [ref-3-6].

 

 

 

• [1] A.J. Cohen, P. Mori-Sanchez, W.T. Yang, Insights into current limitations of density functional theory, Science 321 (2008) 792.
• [2] L. Schimka, J. Harl, A. Stroppa, A. Gruneis, M. Marsman, F. Mittendorfer, G. Kresse, Accurate surface and adsorption energies from many-body perturbation theory, Nature Materials 9 (2010) 741.
• [3] ] D.G. Sangiovanni et al., Ab initio molecular dynamics of atomic-scale surface reactions: insights into metal organic chemical vapor deposition of AlN on graphene, Physical Chemistry Chemical Physics 20 (2018) 17751.
• [4] A. Jamnig et al., Atomic-scale diffusion rates during growth of thin metal films on weakly-interacting substrates, Scientific Reports 9 (2019) 6640.
• [5] A. Fasolino, J.H. Los, M.I. Katsnelson, Intrinsic ripples in graphene, Nature Materials 6 (2007) 858.
• [6] K.V. Zakharchenko et al., Finite Temperature Lattice Properties of Graphene beyond the Quasiharmonic Approximation, Physical Review Letters 102 (2009) 046808.
• [7] J. Son et al., Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor Nature Communications 7, 13261 (2016)

[ref-1]  doi.org/10.1021/acs.jpclett.0c02452; Gerrits, N.; Smeets, E.W.F.; Vuckovic, S.; Powell, A.D.; Doblhoff-Dier, K.; Kroes, G.-J. Density functional theory for molecule–metal surface reactions: When does the generalized gradient approximation get it right, and what to do if it does not. The Journal of Physical Chemistry Letters 2020, 11, 10552-10560

 [ref-2]   doi.org/10.1021/acs.jpcc.9b09774; Rivero Santamaría, A.; Alducin, M.; Díez Muiño, R.; Juaristi, J.I. Ab initio molecular dynamics study of alignment-resolved o2 scattering from highly oriented pyrolytic graphite. The Journal of Physical Chemistry C 2019, 123, 31094-31102

[ref-3] doi: 10.1126/science.1167130; Elias, D.C.; Nair, R.R.; Mohiuddin, T.M.G.; Morozov, S.V.; Blake, P.; Halsall, M.P.; Ferrari, A.C.; Boukhvalov, D.W.; Katsnelson, M.I.; Geim, A.K., et al. Control of graphene's properties by reversible hydrogenation: Evidence for graphane. Science 2009, 323, 610-613.

[ref-4] https://doi.org/10.1002/ese3.833 ; Su, H.; Hu, Y.H. Recent advances in graphene-based materials for fuel cell applications. Energy Science & Engineering n/a.

[ref-5] 10.1016/j.carbon.2018.12.086; Bartolomei, M.; Hernández, M.I.; Campos-Martínez, J.; Hernández-Lamoneda, R. Graphene multi-protonation: A cooperative mechanism for proton permeation. Carbon 2019, 144, 724-730.

[ref-6]  10.1088/1361-648X/aac89f; Bonfanti, M.; Achilli, S.; Martinazzo, R. Sticking of atomic hydrogen on graphene. Journal of Physics: Condensed Matter 2018, 30, 283002

Reviewer 2 Report

The authors are theoretically considered the interaction of the water molecules or ion H+ OH- with graphene and carbon nanotubes. Understanding the properties of strongly confined water is important for a variety of applications such as fast flow and desalination devices, voltage generation, flow sensing, and nanofluidics. Confined water also plays an important role in many biological processes. Using first-principles functional density theory combined with molecular dynamics, the authors develop several simulations on different systems to understand the reactivity of the different carbon surfaces and demonstrate the high affinity of the carbon atom in every situation only for hydrogen ions. As a consequence, the functionalization of the carbon surface upon the presence of a water media is activated by its protonation, then allowing reactivity of anion. It is an important finding in the detailed understanding of graphene or CNT functionalization.

The authors consider the effect of the electrical field on the reactivity of the carbon surface. Electrical filed leads to carbon surface deformation of CHT. But strain effect is not considered or discussed. But typically strain leads to a change in reaction ability. This issue should be discussed.

Generally, after this small correction, the manuscript is suggested for publication.

I would like to note that the authors found a good visual way of presenting the results obtained, which is extremely rare in theoretical studies.

Author Response

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

English language and style

( ) Extensive editing of English language and style required
( ) Moderate English changes required
( ) English language and style are fine/minor spell check required
(x) I don't feel qualified to judge about the English language and style

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	(x)	( )	( )	( )
Is the research design appropriate?	(x)	( )	( )	( )
Are the methods adequately described?	(x)	( )	( )	( )
Are the results clearly presented?	(x)	( )	( )	( )
Are the conclusions supported by the results?	(x)	( )	( )	( )

Comments and Suggestions for Authors

The authors are theoretically considered the interaction of the water molecules or ion H+ OH- with graphene and carbon nanotubes. Understanding the properties of strongly confined water is important for a variety of applications such as fast flow and desalination devices, voltage generation, flow sensing, and nanofluidics. Confined water also plays an important role in many biological processes. Using first-principles functional density theory combined with molecular dynamics, the authors develop several simulations on different systems to understand the reactivity of the different carbon surfaces and demonstrate the high affinity of the carbon atom in every situation only for hydrogen ions. As a consequence, the functionalization of the carbon surface upon the presence of a water media is activated by its protonation, then allowing reactivity of anion. It is an important finding in the detailed understanding of graphene or CNT functionalization.

The authors consider the effect of the electrical field on the reactivity of the carbon surface. Electrical filed leads to carbon surface deformation of CHT. But strain effect is not considered or discussed. But typically strain leads to a change in reaction ability. This issue should be discussed.

Author answer: We thanks the Reviewer for this remark. The strain representing the C-C bond distortion upon adsorption or under the effect of electric field, considerably increases the elasticity of carbon nanotube and enhances the sp³ character of the carbon atoms. Here, the deformation effect is an important factor which could improve the reactivity of the carbon atoms in CNT and graphene. Indeed, it has been shown in previous studies that the tensile strain on a single sheet of graphene can influence the interaction of adsorbents but also make possible the modification of its mechanical and physical properties [1-4].

We analyzed the structural difference between the tube deformed under the effect of 25 eV and an ideal tube by calculating the average deviation between the bond lengths of the two structures. This leads to a standard deviation averaged over all the carbon bonds equal to 0.01085, i.e. 1% deformation of the carbon wall. We added a small discussion about the effect of deformation before figure 4 “The final adsorption … physical properties”.  Line 298-302

 

Generally, after this small correction, the manuscript is suggested for publication.

I would like to note that the authors found a good visual way of presenting the results obtained, which is extremely rare in theoretical studies.

 

 

[1]  http://dx.doi.org/10.1002/cphc. 201100847; Boukhvalov, D.W.; Son, Y.-W. Covalent functionalization of strained graphene. ChemPhysChem 2012, 13, 1463-1469.

[2]  10.1209/0295-5075/100/36005;  Baimova, J.A.; Dmitriev, S.V.; Zhou, Discrete breather clusters in strained graphene. Epl journal 2021, 100, 36005.

[3] https://doi.org/10.1103/PhysRevB.86.035427; Baimova, J.A.; Dmitriev, S.V.; Zhou, K.; Savin, A.V. Unidirectional ripples in strained graphene nanoribbons with clamped edges at zero and finite temperatures. Physical Review B 2012, 86, 035427

[4]  https://doi.org/10.1016/j.physleta.2017.06.017 ; Katin, K.P.; Prudkovskiy, V.S.; Maslov, M.M. Chemisorption of hydrogen atoms and hydroxyl groups on stretched graphene: A coupled qm/qm study. Physics Letters A 2017, 381, 2686-2690.

Reviewer 3 Report

Using density functional theory and molecular dynamics simulations authors study behavior of water molecules on carbon nanostructures subjected to external electric field. The systems they consider include graphene nanoflakes and carbon nanotubes. The authors demonstrate a very strong affinity of any considered surface for hydrogen ions and almost no adsorption of hydroxyl groups. In fact the hydroxyl groups can adsorb on H-functionalized surface. The external electric field acts as a catalyst for the chemisorption of H ions on C surfaces. The presence of dissociated sodium chloride triggers desorption of H+ and OH- ions from graphene layer or reverses the adsorption order of dissociated water ions inside carbon nanotubes.

In my opinion the present study, albeit standard, is interesting in view of the transport of fluid in carbon-based systems or devices. This sheds additional light on the behavior of flat and curved carbon surfaces in contact with water.

My filling is that the manuscript can be accepted for publication in Nanomaterials. However before I will give full recommendation I have a few points which should be addressed by the authors.

1. page 3, lines 92-95: The sentence is unclear. The PBE is a flavor of gradient-corrected correlation-exchange functionals.

2. Please describe the studied system in more details. At present it is unclear in which direction acts the electric field for particular systems. Please also give a magnitude of the electric field in volts per angstrom.

3. page 4, lines 163-167: It is unclear for me what the authors mean by "a defect in the electronic structure of the planar surface". I believe it should be a structural defect. Anyway this sentence also is a sort of speculation. What are the arguments supporting this statement? Perhaps the adsorption of HO- in this case can be explained by charging the surface by H+ ions.

4. Table 4: Comparing adsorption energies, one can conclude that very weak (1 eV) and very strong (25 eV) electric fields have the same effects on the system, while the intermediate fields change properties of the system. What is the explanation for that?

5. page 9, lines 274-276: Authors state that "the hydroxyl remains close to surface part charged in an opposite way", while Figure 4 shows that it adsorb in the neutral part of the system. I believe the statement describes the time prior to the adsorption. This should be explained.

 

Author Response

Open Review

(x) I would not like to sign my review report
( ) I would like to sign my review report

English language and style

( ) Extensive editing of English language and style required
( ) Moderate English changes required
( ) English language and style are fine/minor spell check required
(x) I don't feel qualified to judge about the English language and style

 

	Yes	Can be improved	Must be improved	Not applicable
Does the introduction provide sufficient background and include all relevant references?	(x)	( )	( )	( )
Is the research design appropriate?	( )	(x)	( )	( )
Are the methods adequately described?	(x)	( )	( )	( )
Are the results clearly presented?	( )	(x)	( )	( )
Are the conclusions supported by the results?	(x)	( )	( )	( )

Comments and Suggestions for Authors

Using density functional theory and molecular dynamics simulations authors study behavior of water molecules on carbon nanostructures subjected to external electric field. The systems they consider include graphene nanoflakes and carbon nanotubes. The authors demonstrate a very strong affinity of any considered surface for hydrogen ions and almost no adsorption of hydroxyl groups. In fact the hydroxyl groups can adsorb on H-functionalized surface. The external electric field acts as a catalyst for the chemisorption of H ions on C surfaces. The presence of dissociated sodium chloride triggers desorption of H+ and OH- ions from graphene layer or reverses the adsorption order of dissociated water ions inside carbon nanotubes.

In my opinion the present study, albeit standard, is interesting in view of the transport of fluid in carbon-based systems or devices. This sheds additional light on the behavior of flat and curved carbon surfaces in contact with water.

My feeling is that the manuscript can be accepted for publication in Nanomaterials. However, before I will give full recommendation I have a few points which should be addressed by the authors.

1. page 3, lines 92-95: The sentence is unclear. The PBE is a flavor of gradient-corrected correlation-exchange functionals.

Author answer: Referee is right, our sentence was unclear. We have rewritten it as: “The geometry optimization was performed through the « Open source package for Material eXplorer code » (OpenMX) using a combination of molecular dynamics and Density Functional Theory by way of generalized gradient approximation for the exchange-correlation energy proposed by Perdew, Burke, and Ernzerhof (GGA-PBE)” line 90-93.

 

2. Please describe the studied system in more details. At present it is unclear in which direction acts the electric field for particular systems. Please also give a magnitude of the electric field in volts per angstrom.

Author answer: as asked by the Referee, we have indicated the way to transform the electric field unit in eV to V/Å in the text. (see line 166-169).

We have also amended the text to clarify the direction of the electric field. To be clearer, we have added these details in the text :” the electric field is applied in the direction of the x axis of the elementary cell presented in figure 1c. The studied slabs (CNT and graphene layer) and ESMs are placed parallel to the y-z plane. The electric field is therefore applied perpendicular to the tube axis and to the graphene plane. The effective screening mediums are placed at the cell boundaries conforming to fig. 1c. Also note that the origin of the x-axis is set at the cell boundary”. Line 142 – 145.

3. page 4, lines 163-167: It is unclear for me what the authors mean by "a defect in the electronic structure of the planar surface". I believe it should be a structural defect. Anyway this sentence also is a sort of speculation. What are the arguments supporting this statement? Perhaps the adsorption of HO- in this case can be explained by charging the surface by H+ ions.

Author answer: We thank the referee for this relevant remark. Indeed, the adsorption of H⁺ is at the origin of the adsorption of HO^-. The latter cannot occur spontaneously under the external field effect. It is favored by the imbalance of the charge carriers (electrons and holes) following the adsorption of H⁺. We have modified the text to be more explicit.

4. Table 4: Comparing adsorption energies, one can conclude that very weak (1 eV) and very strong (25 eV) electric fields have the same effects on the system, while the intermediate fields change properties of the system. What is the explanation for that?

Author answer: We thank the Referee for his very interesting remark. In fact, the adsorption of OH^- is also observed at 15eV. We believe that the adsorption of OH^- is highly dependent on the position of the anion during the simulation. If it did not appear at a weak electric field (5 and 10 eV), it is due either to the reformation of water molecule (random) or to a simulation time which did not allow to reach the OH^- observation. Due to the important CPU time of the simulation, we preferred to stop the simulations at the same duration to compare our results. A remark has been added to the text. Line 270-272.

5. page 9, lines 274-276: Authors state that "the hydroxyl remains close to surface part charged in an opposite way", while Figure 4 shows that it adsorbs in the neutral part of the system. I believe the statement describes the time prior to the adsorption. This should be explained.

Author answer: Referee is right. The hydroxyl remains close to the zone presenting an opposite charge before it is adsorbed. The latter is observed on the quite neutral part of the CNT. However, we believe that the adsorption of hydroxyl in this planar zone is mainly due to the consequent mechanic strain. Indeed, it would be extremely difficult to deform the curved zone of the CNT again while the planar zone is more labile. We have corrected the text accordingly. Line 298-302.

Round 2

Reviewer 1 Report

The authors have done a good with revision. The paper is much stronger now.

There is one last issue that the authors should address before publication:

Lines 106-107 on page 3 of the revised text. The text reads: "...accurate in describing molecule−surface interaction, reaction pathways [61], adsorbate diffusion [62]...

The reference [62]: (Fasolino et al., Intrinsic ripples in graphene. Nature materials) does not investigate adsorbate diffusion. The authors should substitute that reference with ref that deal with ab initio MD adatom diffusion on graphene or carbon nanotubes.

A couple of suggestions are:

1) A. Jamnig "Atomic-scale diffusion rates during growth of thin metal films on weakly-interacting substrates" Scientific Reports 9, 6640 (2019)

2) V. Gervilla "Anomalous versus normal room-temperature diffusion of metal adatoms on graphene" The Journal of Physical Chemistry Letters 11, 8930 (2020)

 

Author Response

The authors have done a good with revision. The paper is much stronger now.

There is one last issue that the authors should address before publication:

Lines 106-107 on page 3 of the revised text. The text reads: "...accurate in describing molecule−surface interaction, reaction pathways [61], adsorbate diffusion [62]...

The reference [62]: (Fasolino et al., Intrinsic ripples in graphene. Nature materials) does not investigate adsorbate diffusion. The authors should substitute that reference with ref that deal with ab initio MD adatom diffusion on graphene or carbon nanotubes.

A couple of suggestions are:

1) A. Jamnig "Atomic-scale diffusion rates during growth of thin metal films on weakly-interacting substrates" Scientific Reports 9, 6640 (2019)

2) V. Gervilla "Anomalous versus normal room-temperature diffusion of metal adatoms on graphene" The Journal of Physical Chemistry Letters 11, 8930 (2020)

 

Author Reply: Referee is right. We apologize for our mistake. The reference to Fasolino et al. was erased in this new version. We have replaced it by the 2 references suggested by the Referee