Insights into the Electrochemical Behavior of Mercury on Graphene/SiC Electrodes

Round 1

Reviewer 1 Report

This work reported the fundamental studies of the electrochemical behavior of Mercury on epitaxial graphene. This work would help to better understand the Hg behavior on the graphene-based electrode and for the design of Hg sensors. I recommended it to be accepted after the minor revision.

There are some grammatical mistakes and format errors in the manuscript. For example, line 29, one of the most toxic chemical element, should be elements; line 58, nafion should be Nafion; line 142, 218 and 233, some extra blank should be deleted; line 144, the electron of e, it is better to add the superscript e^-.

The electrolyte used in this study was HClO4, how about other electrolytes, such as HCl, HNO3, H2SO4. Electrolytes are important in understanding the electrochemical behaviors. 

The authors changed the concentration of HClO4 to study the electron transfer step and assigned two kinds of electron steps in different electrolyte concentrations. However, from Figure 3, the reduction peak showed in the higher concentration of HClO4 also look like a two-step electron transfer process as well as the oxidation peak (a shoulder peak can be observed). Therefore, does it suitable or do the authors have other evidence to show it followed a one-step electron transfer process?

Author Response

Dear Referee #1,

Thank you for careful review of our paper. Your comments are very valuable for us. We are grateful for the opportunity to provide you with a new version of our manuscript. Following your advices and comments, we added arguments to justify the using of the HClO₄ buffer as an electrolyte solution and modified the discussion related to the reduction process in the higher concentration of HClO₄. All the changes we did in the manuscript are marked in yellow while the answers below are in red.

Reviewer’s Comment: There are some grammatical mistakes and format errors in the manuscript. For example, line 29, one of the most toxic chemical element, should be elements; line 58, nafion should be Nafion; line 142, 218 and 233, some extra blank should be deleted; line 144, the electron of e, it is better to add the superscript e^-.     

Authors’ Response: We have made required changes.

Reviewer’s Comment: The electrolyte used in this study was HClO4, how about other electrolytes, such as HCl, HNO₃, H₂SO₄. Electrolytes are important in understanding the electrochemical behaviors. 

Authors’ Response: Indeed, we have chosen HClO₄ buffer as a model electrolyte solution to monitor the Hg redox reactions at the epitaxial graphene electrode surface. Our choice can be explained by the fact that, comparing to other chemicals, the perchlorate ions (ClO⁴⁻) exhibit non-complexing character with respect to metal cations in aqueous solutions [1-3]. It means that electrochemical measurements performed in the frames of the current work will provide critical information only on Hg-involved oxidation-reduction reactions, but not on the reactions involving more complicated chemical complexes. On the other hand, a fundamental understanding of the nature of the Hg redox reactions in the presence of other electrolytes namely HCl, HNO₃, H₂SO₄, is of great interest also, but this issue is out of scope of this article.

Reviewer’s Comment: The authors changed the concentration of HClO₄ to study the electron transfer step and assigned two kinds of electron steps in different electrolyte concentrations. However, from Figure 3, the reduction peak showed in the higher concentration of HClO₄ also look like a two-step electron transfer process as well as the oxidation peak (a shoulder peak can be observed). Therefore, does it suitable or do the authors have other evidence to show it followed a one-step electron transfer process?                                         

Authors’ Response: We agree with reviewer that the reduction peak in the higher concentration of HClO₄ is quite asymmetric, possibly indicating two-step electron transfer process. Nevertheless, the oxidation peak has only one component even after several cycles. The contribution of the shoulder peak is negligibly small. Therefore, we believe that the oxidation process is one stage process. According to Reviewer’s comments, we modified the Figure 3b, and related discussions.

References

Yamaguchi, T.; Nomura, M.; Wakita, H.; Ohtaki, H. An extended X-ray absorption fine structure study of aqueous rare earth perchlorate solutions in liquid and glassy states. J. Chem. Phys.1988, 89, 5153–5159.; Sémon, L.; Boehme, C.; Billard, I.; Hennig, C.; Lützenkirchen, K.; Reich, T.; Roßberg, A.; Rossini, I.; Wipff, G. Do Perchlorate and Triflate Anions Bind to the Uranyl Cation in an Acidic Aqueous Medium? A Combined EXAFS and Quantum Mechanical Investigation. ChemPhysChem2001, 2, 591–598.; Binnemans, K. Applications of tetravalent cerium compounds. In Handbook on the Physics and Chemistry of Rare Earths, 1st ed.; Gschneidner, K.A., Jr., Bünzli, J.-C.G., Pecharsky, V.K., Eds.; Elsevier Science Publisher B.V.: Amsterdam, The Netherlands, 2006; Volume 3, pp. 306–307. ISBN 9780080466729.

Author Response File: Author Response.docx

Reviewer 2 Report

Please see the attached document.

Comments for author File: Comments.docx

Author Response

Dear Referee #2,

Thank you for careful review of our paper. Your comments are very valuable for us. We are grateful for the opportunity to provide you with a new version of our manuscript. Following your advices and comments, we added arguments to justify the using of the epitaxial graphene as a working electrode material and modified the discussion section. All the changes we did in the manuscript are marked in yellow while the answers below are in red.

1. Reviewer’s comment: In the Abstract and Introduction, the authors have a claim that electrochemical methods have a potential towards mercury detection. However, most of the further experimental data, the research conditions seem to be very far off from the real life environmental (and fast) detection, such as the mercury comes as a salt and the electrochemical reaction are performed in HClO4. It would be great to provide a vision how those experimental conditions can be transformed into further real life mercury detection. Moreover, the real samples would most possibly be comprised of a large number of other ions, not only Hg. Is there any way to provide selectivity to the method?

Authors’ response:  We would like to emphasize that the aim of the current research is two-fold: to shed light on the nature of the response of epitaxial graphene electrode to Hg exposure and to understand the Hg kinetics on the epitaxial graphene.  These questions have never been studied before and have fundamental and practical values. We did not intend to address environmental detection issues. Although we realize that the pristine epitaxial graphene (EG) cannot be directly used for effective Hg sensing due to the high diffusion coefficient of Hg and weak vdW interaction between Hg and EG, this material can be artificially modified via creation of  additional electroactive sites (carbon vacancies, for instance) to attain desirable sensing performance. As has been shown in our previous work, the carbon vacancies can significantly improve the adsorption energy of Hg on graphene. In this context, knowledge on Hg redox reactions and kinetics onto pristine epitaxial graphene can be considered as a starting point for further investigation and development of EG-based sensing platform for real-time detection of mercury. From a standpoint of the selectivity, information on the correct positions of the Hg-related reduction and oxidation peaks is of great technological importance, since it can be helpful to design sensors, which will be cross-sensitive to specific heavy metals. This is because each metal has a unique redox potential. Therefore, electrochemical measurements (square wave anodic stripping voltammentry, for instance) will enable to distinguish between different metals. To be more specific, in the case of the epitaxial graphene working electrode the corresponding oxidation peaks for three most toxic heavy metals, namely Cd, Hg and Pb are located at -0.79 V, 0.24 V and -0.43 V [1, 2], which creates excellent prerequisites of discriminative analysis without overlapping the redox peaks.  Concerning ”real life environmental (and fast) detection”, it is important to note that using the HClO₄ electrolyte enables, to some extent, mimicking the formation of the hydrated Hg divalent ions in aqueous solution. This is because perchlorate ions (ClO⁴⁻) exhibit non-complexing character with respect to metal cations in aqueous solutions [3-5]. It means that electrochemical measurements performed in the frames of the current work will exceptionally provide critical information only on Hg-involved oxidation-reduction reactions, but not on the reactions involving more complicated chemical complexes. Electrochemical analysis using the Hg salts and the acidic buffers is an initial stage needed to validate the overall idea of Hg sensing prior to analysis of the real water samples.

2. Reviewer’s comment: Why there was no concentration dependent study performed? Authors have used only 0.1mM Hg concentration. Is there a specific reason for that concentration? What is the smallest possible concentration that can be “picked up” with the method?

Authors’ response: The sensitivity of the pristine epitaxial graphene to Hg was very poor. Therefore, to study the Hg redox behavior and kinetics at the epitaxial graphene electrode we used the Hg concentration of 0.1mM, which is high enough to provide intense electrochemical signal.

3. Reviewer’s comment: It is understandable that the introduction and the literature referenced have been specifically tuned to only electrochemical methods. However, it might be great to put a more widespread comparison of the methods, and to introduce a comparison to other methods. Specifically, the works where graphene based FETs are used to amperometrically record presence of Hg should be mentioned and put into comparison [10.1021/nn402702w].

Authors’ response: We added the required information. Please see the following sentence in the revised version of the manuscript: “Apart from the traditional analytical methods for Hg2+ detection (atomic absorption/fluorescence spectroscopy, inductively coupled plasma mass spectrometry, liquid chromatography-mass spectrometry [21]) and highly sensitive amperometric methods exploiting field-effect transistor (FET)-type discriminative sensors [22-25] that usually require complicated sample/device preparation procedures, the electrochemical methods are more convenient ways to facilitate real-time monitoring of mercury at nano-concentrations below permissible level and have therefore attracted widespread attention from the standpoints of their portability and simplicity [26, 27].”

4. Reviewer’s comment: The choice of epitaxial graphene over e.g. CVD grown graphene is not exactly clear. The main question here, is it the unique structure of graphene that provides this detection? Would a bilayer graphene have similar property? Would CVD graphene be different to epitaxial? Would a multilayer graphene be much different? Would graphite be different? Also, would be great to have a direct look into comparison of the graphene(s) and carbon for Hg detection. Maybe a table analyzing the literature would be great.

Authors’ response: The choice of the epitaxial graphene is justified by the fact that this kind of graphene offers a combination of advantages over other graphenes (especially, CVD graphene that has, in most cases, mosaic structure with rotated domains): large surface area, high quality of monolayer graphene, thickness uniformity, wide potential window, high signal-to-noise ratio, transfer-free technology and direct sublimation growth without precursors [6-8].  All these advantages provide excellent prerequisites of development of monolithic wafer-scale sensorics. Unfortunately, the comparative analysis of the graphenes and different carbon-based materials with respect to Hg is out of scope of the current research and requires to be addressed in the frames of a comprehensive review paper. However, to our best knowledge, the thickness of graphene influences significantly the adsorption energy of metals on graphene and charge transfer between them [9, 10].  This is because the interplanar van der Waals interaction between graphene sublayers is also involved into total interaction between graphene and specific metal. Therefore, one can expect different electrochemical signals with respect to Hg depending on the graphene thickness. Furthermore, in the case of graphite, additionally to basal plane the edges of graphite offer extra electrochemically-active sites for Hg adsorption, also affecting the electrochemical signal. As was mentioned before, CVD graphene has mosaic structure and thus a lot of domain boundaries are available for Hg adsorption as active sites. Nevertheless, even though epitaxial graphene has nominally smaller number of electroactive sites compared to graphite and CVD graphene, a high-speed signal readout of the sensor is expected for this two-dimensional material due to its higher conductivity and lower noise in comparison to CVD graphene and multilayer graphene. In this context, a balance between graphene quality and density of the electroactive sites must be reached. We are aimed to study related phenomena in the future.

5. Reviewer’s comment: From the paragraph lines 215-222 it seems that the authors have calculated diffusion coefficient of 6.63x10^-2 cm²/s, and compared to a single other reference [61]. First of all, would be great to compare the value to other works. Second of all, continued discussion has lead the authors to estimate the average number of density of nucleation states as 5.59x10³ cm^-2. How was the calculation performed? Please also give perspective towards this number, how is it different from other material.

Authors’ response: We added the required information. We determined the diffusion coefficient of mercury by using chronoamperometry method and applying the Scharifker-Hills approach. In this regard, we cannot provide more comparative literature data due to the limited number of chronoamperometric studies on Hg at working electrode. In this regard, there are, at least, two reports (we have already mentioned in the paper) on Hg electrodeposition on boron-doped diamond electrode [11] and vitreous carbon electrode [12], which provide the diffusion coefficient of Hg. The density of the nucleation sites was determined by using the formula proposed in Ref[13].

High diffusion coefficient of Hg and low number of active nucleation sites originating from weak vdW interaction between Hg and EG do not promote fast charge transfer reactions at the electrode surface and thus the sensitivity of the pristine epitaxial graphene with respect to the Hg is expected to be poor.  Therefore, this material needs to be modified possibly through introduction of additional electroactive sites (carbon vacancies, for instance) to attain desirable sensing performance, but maintaining the graphene quality as high as possible.

6. Reviewer’s comment: Figure 5 is not mentioned in the text at all.

Authors’ response: We made required changes.

7. Reviewer’s comment: The great numerical models, Figures 6-8 seem to be rather stand alone in their placement within the work. I believe the authors should put extra links between the numerical models and experimental findings, as well as to give perspective for future works.

Authors’ response: We added the required information. First, by performing DFT calculations we revealed that the interaction of reduced Hg species with graphene surface occurs mainly through weak van der Waals interaction. This finding explains experimentally derived high diffusion coefficient of Hg and low number of active nucleation sites. Due to the weak interaction, Hg species can freely migrate across graphene surface without energy barriers. Second, we modeled the realistic reduction process considering the Hg divalent ion with first hydration shell and found that added electrons are completely employed to reduce the charge of the Hg ion. This shows the principal possibility of Hg electroreduction at the graphene surface .

Minor:

Reviewer’s comment: Please provide the area of the active site of graphene that is exposed to electrolyte.

Authors’ response: We added the required information. The electrochemical reactions occur at the area of 3.1 mm².

Reviewer’s comment: Please provide the values of Hg and HClO4 concentrations directly onto Fig. 2 and Fig. 4 (similarly as it is done in Fig. 1 and Fig. 3).

Authors’ response: We made required changes.

Reviewer’s comment: Figure 1 consists of three blue curves. Please give explanation (maybe add arrows) to explain the difference between those.

Authors’ response: We made required changes. Three blue curves correspond to different cycles during cyclic voltammetry measurements.

Reviewer’s comment: Fig. 2a. Instead of the meaningless values of area, please provide the values of charge density provided in the manuscript body, to simplify reader’s perception of the data.

Authors’ response: We made required changes.

Reviewer’s comment: Table 1. The potential of anodic process is given as “negative” -0.244V, which contradicts to the data and discussion above.

Authors’ response: We corrected this typo. Indeed, the correct value of potential is positive.

Reviewer’s comment: Figure 4a. Discussion of the current increase/decrease with increased amplitudes of potential charges is confusing. To my understanding, the current density is approximately -6 mA/cm2 at -0.2V, and -4.5 mA/cm2 at -0.3V. The -6 is larger current than -4.5, which contradicts to your discussion on lines 191-193.

Authors’ response: We made required changes. At the initial stages,  parameter increases (which is associated with enlarging surface density of Hg nuclei) and then decreases gradually after it reaches some maximum dependent on the applied potential. During the early stages of Hg nucleation, the time corresponding to the maximum value of the  parameter shortens with the increase in the deposition potential.

Reviewer’s comment: Origin, as well as the general importance of Formula (6) is completely unknown. Where are the numbers in the formula taken from? Is it empirical formula that evolved through fitting the experimental data, or somewhat general?

Authors’ response: The formula (6) is not empirical. This is simplified representation of the classical Scharifker-Hills equations [13], which is commonly used [14] to describe the models for instantaneous and progressive nucleation.

Reviewer’s comment: The numerical models assumed a “single layer graphene”. Would a bilayer graphene, multilayer graphene, or even a graphite surface give a completely different interaction plots?

Authors’ response: Interplanar van der Waals interaction between graphene sublayers in multilayer graphene and graphite will influence the total interaction between Hg and topmost graphene layer and thus different interaction plots are expected. 

References

Shtepliuk, I.; Santangelo, M.F.; Vagin, M.; Ivanov, I.G.; Khranovskyy, V.; Iakimov, T.; Eriksson, J.; Yakimova, R. Understanding Graphene Response to Neutral and Charged Lead Species: Theory and Experiment. Materials2018, 11, 2059. Shtepliuk, I.; Vagin, M.; Iakimov, T.; Yakimova, R. Fundamentals of Environmental Monitoring of Heavy Metals Using Graphene, Chemical Engineering Transactions 2019, 73, 7-12. Yamaguchi, T.; Nomura, M.; Wakita, H.; Ohtaki, H. An extended X-ray absorption fine structure study of aqueous rare earth perchlorate solutions in liquid and glassy states. J. Chem. Phys.1988, 89, 5153–5159.; Sémon, L.; Boehme, C.; Billard, I.; Hennig, C.; Lützenkirchen, K.; Reich, T.; Roßberg, A.; Rossini, I.; Wipff, G. Do Perchlorate and Triflate Anions Bind to the Uranyl Cation in an Acidic Aqueous Medium? A Combined EXAFS and Quantum Mechanical Investigation. ChemPhysChem2001, 2, 591–598.; Binnemans, K. Applications of tetravalent cerium compounds. In Handbook on the Physics and Chemistry of Rare Earths, 1st ed.; Gschneidner, K.A., Jr., Bünzli, J.-C.G., Pecharsky, V.K., Eds.; Elsevier Science Publisher B.V.: Amsterdam, The Netherlands, 2006; Volume 3, pp. 306–307. ISBN 9780080466729. Shtepliuk, I.; Khranovskyy, V.; Yakimova, R. Combining graphene with silicon carbide: Synthesis and properties—A review. Semicond. Sci. Technol. 2016, 31, 113004. Virojanadara, C.; Syväjarvi, M.; Yakimova, R.; Johansson, L.; Zakharov, A.; Balasubramanian, T. Homogeneous large-area graphene layer growth on 6H-SiC(0001). Phys. Rev. B2008, 78, 245403. Yazdi, G.R.; Iakimov, T.; Yakimova, R. Epitaxial Graphene on SiC: A Review of Growth and Characterization. Crystals2016, 6, 53. Hardcastle, P.; Seabourne, C.R.; Zan, R.; Brydson, R.M.D.; Bangert, U.; Ramasse, Q.M.; Novoselov, K.S.; Scott, A.J. Mobile metal adatoms on single layer, bilayer, and trilayer graphene: An ab initio DFT study with van der Waals corrections correlated with electron microscopy data. Phys. Rev. B2013, 87, 195430. Shtepliuk, I.; Yakimova, R. Interband Absorption in Few-Layer Graphene Quantum Dots: Effect of Heavy Metals. Materials2018, 11, 1217. Vinokur, N.; Miller, B.;Avyigal, Y.; Kalish, R. Cathodic and Anodic Deposition of Mercury and Silver at Boron-Doped Diamond Electrodes.J. Electrochem. Soc.1999, 146 (1) 125-130.doi: 10.1149/1.1391574 Serruya, A.;Mostany, J.;Scharifker, B.R. The kinetics of mercury nucleation from Hg₂²⁺ and Hg²⁺ solutions on vitreous carbon electrodes.J. Electroanal. Chem. 1999, 464, 39–47. doi: 10.1016/S0022-0728(98)00464-1 Scharifker,B.; Hills, G. Theoretical and experimental studies of multiple nucleation. Electrochim. Acta1983, 28, 879.doi: 10.1016/0013-4686(83)85163-9 Grujicic, D.; Pesic, B. Electrodeposition of copper: the nucleation mechanisms. Electrochim. Acta 2002, 472901-2912

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors added new information to describe their work which I think they answered my question. I suggest it be accepted as the present form.

Reviewer 2 Report

The authors have addressed all the questions and I would happily suggest the manuscript for publication.