This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2018, 3, 17700−17706
http://pubs.acs.org/journal/acsodf
Fluorographenes for Energy and Sensing Application: The Amount of Fluorine Matters Chee Shan Lim,† Zdenek Sofer,‡ Jan Plutnar,‡ and Martin Pumera*,‡,§
Downloaded via 146.185.205.234 on December 24, 2018 at 14:53:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 ‡ Center for the Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic § Center for Flexible Wearable Electronics, Central European Institute of Technology, Brno University of Technology, Purkyňova 123, 613 00 Brno, Czech Republic ABSTRACT: With graphene and its derivatives playing important roles in a wide array of applications lately, modification and optimization of these materials become of paramount importance. In this work, we study the electrochemical and electrocatalytic behaviors of three fluorinated graphenes with varied fluorine contents. Unsurprisingly, the fluorinated graphene materials have displayed enhanced electrochemical sensing in various biomarkers, including uric acid, ascorbic acid, and dopamine, and also in energy applications, such as hydrogen evolution and oxygen reduction over the bare glassy carbon electrode surface. A comparison within the fluorinated graphenes showed that the materials with the higher fluorine level produced the best performance electrochemically and electrocatalytically in general.
■
chemical exfoliation of graphite fluoride16 and has also been regarded as the 2D counterpart of Teflon.27 In recent years, fluorinated graphene has been highly recognized for its improved optical and electron transport properties from its parent graphene29,30 and used as sorbent for gas separation.31 Despite the heightened impetus to study this new derivative of graphene, there have been limited studies on its potential in electrochemical applications. As prior research has shown promising behavior in electrochemical and electrocatalytic studies using fluorinated graphite,32,25 we wish to further extend the field of study to fluorinated graphenes (F-graphene). In this work, three fluorinated graphenes, namely, CF0.02, CF1.02, and CF1.39 were studied; they differ by their level of fluorination and are abbreviated by the carbon-to-fluorine (C/F) ratios. As properties of halogenated graphenes can be adjusted by tuning the level of halogenations,33 the effect of degree of fluorination on sensing is examined with important biomarkers, namely, uric acid, ascorbic acid, and dopamine; this effect is also affirmed by comparing the HET rates. Apart from electrochemical sensing, the viability of these fluorinated graphenes as electrocatalysts for hydrogen evolution (HER) and oxygen reduction (ORR) is also investigated. It was observed that an increase in the degree of fluorination gave rise to better sensing and electrocatalytic properties in general, asserting the
INTRODUCTION Graphene is a one-atom thick, two-dimensional (2D) semiconductor with sp2-hybridized carbon arranged in a honeycomb network.1 The absence of band gap,2 coupled with excellent electrical3 and electron transfer properties4 has allowed graphene to enjoy high prestige and recognition in various areas, including catalysis and biosensing.5,6 Despite the numerous remarkable applications graphene is involved in, further efforts have been carried out to improve the behavior and abilities of graphene via modification processes. More popular modification routes include reduction treatments, and addition of other atoms such as metals has given rise to enhanced properties and wider range of applications of graphene.7−12 Modified graphene derivatives have also made their mark in energy and storage applications over the years.13−15 One of the commonly used modification pathways recently is the halogenation of graphene,16 which can be performed using thermal annealing in the presence of a halogen dopant or exfoliation of graphene in a halogen atmosphere.17−21 The resulting halogenated graphene possesses an sp2-hybridized planar carbon network as well as sp3-hybridized carbons covalently attached to the halogens22 and exhibits interesting properties, including fast heterogeneous electron transfer (HET) rate and an open band gap.23−28 Of the few halogenated graphenes studied, fluorinated graphene has become more prominently used lately due to the increased reactivity of fluorine over that of the other halogens. Fluorinated graphene is typically fabricated via mechanical or © 2018 American Chemical Society
Received: September 29, 2018 Accepted: December 5, 2018 Published: December 19, 2018 17700
DOI: 10.1021/acsomega.8b02582 ACS Omega 2018, 3, 17700−17706
ACS Omega
Article
advantages of halogenated materials in the field of electrochemistry.
■
RESULTS AND DISCUSSION Characterization of the three F-graphene materials has been carried out using elemental combustible analysis (with F−-ion selective electrode), X-ray spectroscopy (XPS), and energydispersive X-ray spectroscopy to determine the halogen content and identify the presence of other elements in each material. The detailed spectra are presented in our previous work; here, we state the summary of the analysis in Table 1.39 Table 1. Elemental Composition of the Materials as per Combustible Analysis39 a material
carbon
fluorine
oxygen
hydrogen
CF0.02 CF1.02 CF1.39
74.6 46.7 36.4
1.5 42.6 50.7
15.7 3.2 5.2
8.1 7.3 2.8
a
The rest to 100% is N. All data are in % atom.
By fixing the stoichiometry of carbon as 1, the carbon-tofluorine (C/F) ratios of the materials were found to be 1:0.02, 1:1.02, and 1:1.39, respectively. These materials are hence referred to as CF0.02, CF1.02, and CF1.39 throughout this work. The varied oxygen contents of the materials are noteworthy as well, wherein CF1.39 has the largest carbon-to-oxygen (C/O) ratio of 7.04, followed by CF0.02 (4.76) and CF1.02 (2.77).39 The inherent electrochemistry (that is, redox behavior of fluorographene-based electrode itself without added depolarizer) of the three fluorinated graphenes was first examined to establish any oxidizable or reducible species in the materials prior to further electrochemical studies. An apparent anodic peak at +0.30 V, which most likely corresponds to the oxidation of graphene was detected for all of the F-graphene samples, as shown in Figure 1. The graphene network might be the most extensive for CF1.02, as the oxidation peak is most prominent. Another distinct oxidation signal at around +1.20 V was also observed for CF1.02. This peak is almost negligible for the remaining F-graphene materials, implying a possible presence of other impurities in the CF1.02 sample. On the contrary, the reduction signals observed at about −0.75 V can be attributed to the reduction of oxygen, which is present in the graphene materials. The most intense peak was seen for CF0.02, fully substantiating the highest oxygen content recorded upon characterization. As oxidizable and reducible species are present intrinsically, the F-graphene materials are considered electrochemically active and preliminarily suitable for electrochemical studies. The electrochemical behavior of the F-graphene materials, along with the effect of fluorination is first assessed by the HET rates with two redox probes, potassium ferro/ferricyanide (Fe2+/Fe3+) and hexaammineruthenium(III) chloride (Ru2+/ Ru3+). Figure 2 illustrates the HET results when Fe2+/Fe3+, a surface-sensitive probe was used as the electrolyte. Between the F-graphene and bare glassy carbon (GC) surfaces, it is evident that F-graphene-modified surfaces induced a smaller ΔEp−p value, which exhibits an inverse relationship with HET rates (k°), and generated larger currents as compared with the bare GC surface. This is indicative of the potential electrochemical performance of the F-graphene materials, a characteristic displayed by typical graphene and fluorinated graphite
Figure 1. Cyclic voltammograms of the potassium phosphate (PBS) background electrolyte (50 mM, pH 7.2) on the fluorinated graphene surfaces for five consecutive scans in the anodic direction. Conditions: scan rate 100 mV s−1. No analyte was added.
materials. Further analysis reveals smaller ΔEp−p values with increasing levels of fluorination, as seen from Figure 2. The k° values, derived by the Nicholson’s method,34 showed distinct enhancements from 8.61 × 10−4 to 2.42 × 10−3 cm s−1 and finally 3.09 × 10−3 cm s−1 for CF0.02, CF1.02, and CF1.39, respectively. The results suggest improved electrochemical behavior with higher fluorine content in the graphene materials. This is likely related to the changes in the electron density in fluorographene as well as to the fact that with increasing fluorination, the number of defect sites increases. Subsequently, the Ru2+/Ru3+ probe was used for further analysis. Figure 3 shows negligible deviations in ΔEp−p values among the four surfaces, wherein no significant implications 17701
DOI: 10.1021/acsomega.8b02582 ACS Omega 2018, 3, 17700−17706
ACS Omega
Article
studies were performed to uncover the sensing abilities of these F-graphene materials. Three important biomarkers were chosen, namely, uric acid, ascorbic acid, and dopamine, as displayed in Figure 4. A consistent finding across all three
Figure 2. (A) Cyclic voltammograms of 10 mM ferro/ferricyanide on F-graphene and bare GC surfaces and (B) their respective peak-topeak separation ΔEp−p. Conditions: 50 mM PBS background electrolyte, scan rate of 100 mV s−1.
Figure 3. Cyclic voltammograms of 5 mM hexaammineruthenium(III) chloride on F-graphene and bare GC surfaces. Conditions: 50 mM PBS background electrolyte, scan rate 100 mV s−1.
Figure 4. Cyclic voltammograms of 10 mM (A) uric acid, (B) ascorbic acid, and (C) dopamine on the fluorinated graphene and bare GC surfaces. Conditions: 50 mM PBS background electrolyte, scan rate of 100 mV s−1.
were identified when different electrode surfaces were studied. This, however, is not an indication of poor electrochemical behavior of the F-graphene materials, as the Ru2+/Ru3+ probe is known for not being a surface-sensitive probe from studies carried out by McCreery and co-workers, and even the edge and basal plane of graphite shows the same electroactivity with Ru2+/Ru3+ probe.35 With the effect of the degree of fluorination on HET properties of the fluorinated graphene established, subsequent
biomarkers is the superiority in sensitivity of the F-graphene materials over the bare GC surface, judging from the earlier oxidation peak potentials recorded. For uric acid, CF0.02 achieved an oxidation potential of +399 mV whereas CF1.02 and CF1.39 recorded smaller values of +373 and +369 mV, respectively. Despite a marginal difference of 4 mV between CF1.02 and CF1.39, it remains evident that a higher fluorine proportion was able to lower the overpotential required to drive the oxidation of uric acid. The same conclusion can be 17702
DOI: 10.1021/acsomega.8b02582 ACS Omega 2018, 3, 17700−17706
ACS Omega
Article
the significance of fluorine, where electrochemical and electrocatalytic performances for the materials are proportional to the fluorine level in general. A second parameter, Tafel slope values, was also evaluated to provide a better assessment of the materials. The Tafel slopes, along with the calculated values, are displayed in Figure 6 and
drawn for ascorbic acid, wherein CF1.39 generated the earliest oxidation potential of +253 mV whereas CF0.02 attained a much later potential of +353 mV. Noteworthy to mention in this instance is the onset potential of the F-graphene materials for the oxidation of ascorbic acid, which is significantly earlier, about 200 mV, than that achieved for the bare GC surface. While the direct relationship between the level of fluorine and sensing behavior of the graphenes is exemplified by the oxidation of uric acid and ascorbic acid, this trend was not manifested in the case of dopamine. Although the CF0.02modified surface recorded the largest overpotential (320 mV) analogously to the two other biomarkers, CF1.02 achieved the earliest peak potential of +263 mV amongst the three Fgraphene surfaces. A very similar peak potential of +273 mV was, however, recorded for CF1.39; this highlights the importance of the halogen in electrochemical sensing in general. Although sensing performances of the F-graphene materials are not fully determined by the degree of fluorination and other factors including the graphene platform may play a more dominant role for in the case of dopamine, an increase in fluorine content typically enhances the sensing performance and surface sensitivity of the graphene materials. The effect of the fluorine content in the graphene materials was further extended to electrocatalytic studies, wherein the materials were assessed and compared for their potentials in catalyzing two important energy reactions, HER and ORR. Results for HER exhibit general catalytic behavior of the Fgraphene materials, as manifested by the earlier onset potentials (between −0.8 and −0.9 V) compared to those of the bare GC surface in Figure 5. As the onset potential is an
Figure 6. (A) Tafel slopes of hydrogen evolution on the five surfaces and (B) their respective Tafel slope values.
unsurprisingly, the Pt/C surface generated the smallest slope of about 30 mV dec−1. This suggests that a small increase in overpotential was sufficient to induce a 10-fold increment of the current density obtained. CF1.39 generated the next smallest value of 107 mV dec−1; this is about 30 mV dec−1 lower than that of the other two F-G counterparts, again proving the dominance of fluorine content in catalyzing the reaction. Interestingly, the Tafel value of CF0.02 is slightly smaller than that of CF1.02 despite the lower fluorine content. This outcome might be attributed to the amount of oxygen-containing groups instead, which have been believed to hinder the aromaticity of graphene and thereby the activity of graphene-based materials.37 This might hence affect the electrocatalytic activity of CF1.02, in view of most oxygen functionalities present among the F-graphene materials. Despite the small discrepancy, the inferiority of bare GC surface (165 mV dec−1) shows the possibility to explore F-graphene materials as new catalyst alternatives for hydrogen evolution and the comparison among the three F-graphene materials offers new insights to improved catalysis via halogen modifications of the materials. A final study carried out to examine the electrocatalytic behavior of the F-graphene materials on ORR is presented in
Figure 5. Linear sweep voltammograms of hydrogen evolution of the F-G materials compared with Pt/C and bare GC surfaces. Conditions: 0.5 M sulfuric acid, scan rate of 2 mV s−1.
ambiguous factor for the assessment of the total electrode activity, the overpotentials required for the current density value to reach −10 mA cm−2 were recorded and compared.36 Apart from Pt/C which required a very small overpotential of 100 mV, about 900−1000 mV was necessary for the Fgraphene materials to achieve the desired current density value. Despite their inability to rival the performance of Pt/C, the electrocatalytic properties of the fluorinated graphene materials surpassed those of bare GC, indicating an enhancement in the rate of hydrogen evolution. Among the three graphene materials, CF1.39 required the smallest overpotential (∼900 mV), followed by CF1.02 and CF0.02. This trend further affirms 17703
DOI: 10.1021/acsomega.8b02582 ACS Omega 2018, 3, 17700−17706
ACS Omega
■
Figure 7; dotted curves in Figure 7 represent experiments carried out in purged nitrogen atmosphere to confirm the
Article
CONCLUSIONS
The attractiveness of graphene, although discovered and intensively explored for the past few years, has not diminished due to consistent fabrication of new, modified graphene derivatives. In this work, fluorinated graphenes and their electrochemical properties were investigated; the impact of different fluorine levels was also examined. Studies have shown that the F-graphene materials were capable of enhancing heterogeneous electron transfer and detection of important biomarkers electrochemically. Additional studies on the electrocatalysis of hydrogen evolution and oxygen reduction reactions have further highlighted the use of these materials, where they were catalytically active and viable as potential replacements for these energy applications. Detailed assessment of these F-graphene materials suggested a general trend wherein an increase in the degree of fluorination leads to a better electrochemical performance. The HET rate and the potentials required for the oxidation of uric acid and ascorbic acid were augmented with higher fluorine content, whereas HER was also catalyzed at the fastest rate with the CF1.39 surface. Although exceptions were observed for the detection of dopamine and the catalysis of oxygen reduction as CF1.02 outperformed the other two F-graphene materials, the negligible deviations recorded between CF1.02 and CF1.39 certainly affirm the significance of fluorine in improving the electrochemical properties of graphene.
■
EXPERIMENTAL SECTION Materials. N,N-dimethylformamide (DMF), potassium phosphate monobasic, sodium phosphate dibasic, potassium chloride, sodium chloride, uric acid, ascorbic acid, dopamine, potassium hexacyanoferrate(II) trihydrate, potassium hexacyanoferrate(III), sulfuric acid (95−98%, v/v), and potassium hydroxide were purchased from Sigma-Aldrich, Singapore. Glassy carbon (GC) electrodes with a diameter of 3 mm were obtained from Autolab, The Netherlands. Distilled water with a resistivity of 18.2 MΩ cm was used throughout the experiments. Instrumentation. Cyclic voltammetry and linear sweep voltammetry measurements were carried out with a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a personal computer and operated by NOVA 1.10 software. All experiments were performed in a 5 mL electrochemical cell at room temperature using a threeelectrode configuration; a platinum electrode functioned as a counter electrode, whereas an Ag/AgCl (saturated) electrode served as the reference electrode. All electrochemical potentials in this work are presented vs the Ag/AgCl reference electrode unless otherwise stated. Synthesis and Procedures. Fluorinated graphene was prepared from graphene oxide reduced in microwave plasma. Fluorination was performed using N2/F2 mixture with 20% v/v of F2. Graphene (0.5 g) was placed in the autoclave under N2/ F2 atmosphere at 3 bar pressure. Various times and pressures were used to fluorinate graphene to different degrees. The amount of fluorine at the graphene backbone was determined by combustible elemental analysis (using fluorine-ion sensitive electrode) and XPS;39 graphene oxide (without microwave treatment) exposed to 180 °C for 4 days yielded into fluorographene with summary formula CF0.02. Fluorographene with summary formula CF1.02 and CF1.39 was prepared by direct fluorination of graphene (synthesized by microwave
Figure 7. (A) Linear sweep voltammograms of oxygen reduction on the F-graphene, Pt/C, and bare GC surfaces and (B) their respective onset potential values. Conditions: 0.1 M potassium hydroxide, scan rate of 5 mV s−1.
position of the actual oxygen reduction signals. As shown in Figure 7, the earliest onset potential of ORR (fixed at position where 10% of the peak current was attained) was recorded for the Pt/C surface, at around −257 mV. However, contrary to the trends exhibited for the prior studies, CF1.02 achieved the earliest onset potential among the three F-graphene materials, which was about −268 mV. The onset potentials for the remaining two materials were approximately −270 mV. Although the deviations among the values were marginal, as small as 3 mV, the rate of ORR proved to be largely independent of the fluorine content, especially since CF0.02 and CF1.39 obtained almost identical results. Apart from the comparison, promising catalytic behaviors on ORR are evident for the F-graphene materials, wherein the performance surpassed that of the bare GC surface and only fell short of that of Pt/C marginally. Another interesting observation is the currents of the voltammograms, which are indications of the charge transferred during ORR; bare GC and CF1.02 generated the smallest and largest current values, respectively. The large amount of charge transferred for CF1.02 can be attributed to the presence of a significant amount of oxygen-containing groups in the material inherently, as justified by the XPS results.39 It should be noted that the F-graphene is thermally stable to 300 °C.38 17704
DOI: 10.1021/acsomega.8b02582 ACS Omega 2018, 3, 17700−17706
ACS Omega
Article
(9) Subrahmanyam, K. S.; Manna, A. K.; Pati, S. K.; Rao, C. N. R. A Study of Graphene Decorated with Metal Nanoparticles. Chem. Phys. Lett. 2010, 497, 70−75. (10) Xu, H.; Yan, B.; Li, S.; Wang, J.; Wang, C.; Guo, J.; Du, Y. NDoped Graphene Supported PtAu/Pt Intermetallic Core/Dendritic Shell Nanocrystals for Efficient Electrocatalytic Oxidation of Formic Acid. Chem. Eng. J. 2018, 334, 2638−2646. (11) Xu, H.; Yan, B.; Zhang, K.; Wang, J.; Li, S.; Wang, C.; Shiraishi, Y.; Du, Y.; Yang, P. Ultrasonic-assisted synthesis of N-doped graphene-supported binary PdAu nanoflowers for enhanced electrooxidation of ethylene glycol and glycerol. Electrochim. Acta 2017, 245, 227−236. (12) Xu, H.; Yan, B.; Li, S.; Wang, J.; Wang, C.; Guo, J.; Du, Y. Facile Construction of N-Doped Graphene Supported Hollow PtAg Nanodendrites as Highly Efficient Electrocatalysts toward Formic Acid Oxidation Reaction. ACS Sustainable Chem. Eng. 2018, 6, 609− 617. (13) Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321−1326. (14) Chen, D.; Zhao, X.; Chen, S.; Li, H.; Fu, X.; Wu, Q.; Li, S.; Li, Y.; Su, B.-L.; Ruoff, R. S. One-Pot Fabrication of FePt/Reduced Graphene Oxide Composites as Highly Active and Stable Electrocatalysts for the Oxygen Reduction Reaction. Carbon 2014, 68, 755− 762. (15) Lim, C. S.; Sofer, Z.; Toh, R. J.; Eng, A. Y. S.; Luxa, J.; Pumera, M. Iridium- and Osmium-Decorated Reduced Graphenes as Promising Catalysts for Hydrogen Evolution. ChemPhysChem 2015, 16, 1898−1905. (16) Karlický, F.; Kumara Ramanatha Datta, K.; Otyepka, M.; Zbořil, R. Halogenated Graphenes: Rapidly Growing Family of Graphene Derivatives. ACS Nano 2013, 7, 6434−6464. (17) Chronopoulos, D. D.; Bakandritsos, A.; Pykal, M.; Zbořil, R.; Otyepka, M. Chemistry, Properties, and Applications of Fluorographene. Appl. Mater. Today 2017, 9, 60−70. (18) Zbořil, R.; Karlický, F.; Bourlinos, A. B.; Steriotis, T. A.; Stubos, A. K.; Georgakilas, V.; Š afárǒ vá, K.; Jančík, D.; Trapalis, C.; Otyepka, M. Graphene Fluoride: A Stable Stoichiometric Graphene Derivative and Its Chemical Conversion to Graphene. Small 2010, 6, 2885− 2891. (19) Yao, Z.; Nie, H.; Yang, Z.; Zhou, X.; Liu, Z.; Huang, S. Catalyst-Free Synthesis of Iodine-Doped Graphenevia a Facile Thermal Annealing Process and Its Use for Electrocatalytic Oxygen Reduction in an Alkaline Medium. Chem. Commun. 2012, 48, 1027− 1029. (20) Poh, H. L.; Š imek, P.; Sofer, Z.; Pumera, M. Halogenation of Graphene with Chlorine, Bromine, or Iodine by Exfoliation in a Halogen Atmosphere. Chem. - Eur. J. 2013, 19, 2655−2662. (21) Pumera, M.; Sofer, Z. Towards Stoichiometric Analogues of Graphene: Graphane, Fluorographene, Graphol, Graphene Acid and Others. Chem. Soc. Rev. 2017, 46, 4450−4463. (22) Teo, W. Z.; Chng, E. L. K.; Sofer, Z.; Pumera, M. Cytotoxicity of Halogenated Graphenes. Nanoscale 2014, 6, 1173−1180. (23) Urbanová, V.; Karlický, F.; Matěj, A.; Š embera, F.; Janoušek, Z.; Perman, J. A.; Ranc, V.; Č épe, K.; Michl, J.; Otyepka, M.; et al. Fluorinated Graphenes as Advanced Biosensors − Effect of Fluorine Coverage on Electron Transfer Properties and Adsorption of Biomolecules. Nanoscale 2016, 8, 12134−12142. (24) Yuan, S.; Rösner, M.; Schulz, A.; Wehling, T. O.; Katsnelson, M. I. Electronic Structures and Optical Properties of Partially and Fully Fluorinated Graphene. Phys. Rev. Lett. 2015, 114, No. 047403. (25) Chia, X.; Ambrosi, A.; Otyepka, M.; Zbořil, R.; Pumera, M. Fluorographites (CFx)n Exhibit Improved Heterogeneous ElectronTransfer Rates with Increasing Level of Fluorination: Towards the Sensing of Biomolecules. Chem. - Eur. J. 2014, 20, 6665−6671. (26) Karlický, F.; Otyepka, M. Band Gaps and Optical Spectra of Chlorographene, Fluorographene and Graphane from G0W0, GW0 and GW Calculations on Top of PBE and HSE06 Orbitals. J. Chem. Theory Comput. 2013, 9, 4155−4164.
exfoliation in hydrogen plasma) for 1 day and 4 days, respectively. Materials were characterized in a previous publication.39 The code numbers in ref 39 correspond to the materials here as follows: CF0.02 (code number F-G595), CF1.02 (code number F-G596), and CF1.39 (code number FG597). Cyclic voltammetry measurements were recorded at a scan rate 0.1 V s−1 for inherent electrochemistry, HET, and sensing studies. Linear sweep voltammetry measurements were performed at a scan rate 0.002 V s−1 for hydrogen evolution reaction in 0.5 M sulfuric acid and 0.005 V s−1 oxygen reduction reaction in 0.1 M potassium hydroxide, respectively. The reproducibility of the measurements was achieved over 3 experiments, using a different electrode unit each time. The GC electrode surfaces were renewed by polishing with 0.05 μm alumina powder on a polishing pad. A suspension of the desired material with a concentration 1 mg mL−1 in DMF was first prepared, followed by a 60 min ultrasonication. Subsequently, a 1 μL aliquot of the appropriate suspension was deposited onto the electrode surface using a micropipette to immobilize the material onto the working electrode. A randomly distributed film was obtained on the glassy carbon electrode surface upon evaporation of the solvent at room temperature prior to electrochemical scans.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zdenek Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). Z.S. was supported by Czech Science Foundation (GACR No. 17-05421S). This work was created with the financial support of the Neuron Foundation for science support.
■
REFERENCES
(1) Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132−145. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (3) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of SingleLayer Graphene. Nano Lett. 2008, 8, 902−907. (4) Pumera, M. Voltammetry of Carbon Nanotubes and Graphenes: Excitement, Disappointment, and Reality. Chem. Rec. 2012, 12, 201− 213. (5) Huang, C.; Bai, H.; Li, C.; Shi, G. A Graphene Oxide/ Hemoglobin Composite Hydrogel for Enzymatic Catalysis in Organic Solvents. Chem. Commun. 2011, 47, 4962−4964. (6) Bonanni, A.; Pumera, M. Graphene Platform for Hairpin-DNABased Impedimetric Genosensing. ACS Nano 2011, 5, 2356−2361. (7) Liu, J.; Tang, J.; Gooding, J. J. Strategies for Chemical Modification of Graphene and Applications of Chemically Modified Graphene. J. Mater. Chem. 2012, 22, 12435−12452. (8) Lv, R.; Terrones, M. Towards New Graphene Materials: Doped Graphene Sheets and Nanoribbons. Mater. Lett. 2012, 78, 209−218. 17705
DOI: 10.1021/acsomega.8b02582 ACS Omega 2018, 3, 17700−17706
ACS Omega
Article
(27) Dubecký, M.; Otyepková, E.; Lazar, P.; Karlický, F.; Petr, M.; Č épe, K.; Banás,̌ P.; Zbořil, R.; Otyepka, M. Reactivity of Fluorographene: A Facile Way toward Graphene Derivatives. J. Phys. Chem. Lett. 2015, 6, 1430−1434. (28) Otyepková, E.; Lazar, P.; Č épe, K.; Tomanec, O.; Otyepka, M. Organic Adsorbates Have Higher Affinities to Fluorographene than to Graphene. Appl. Mater. Today 2016, 5, 142−149. (29) Robinson, J. T.; Burgess, J. S.; Junkermeier, C. E.; Badescu, S. C.; Reinecke, T. L.; Perkins, F. K.; Zalalutdniov, M. K.; Baldwin, J. W.; Culbertson, J. C.; Sheehan, P. E.; et al. Properties of Fluorinated Graphene Films. Nano Lett. 2010, 10, 3001−3005 https . (30) Withers, F.; Dubois, M.; Savchenko, A. K. Electron Properties of Fluorinated Single-Layer Graphene Transistors. Phys. Rev. B 2010, 82, No. 073403. (31) Schrier, J. Fluorinated and Nanoporous Graphene Materials As Sorbents for Gas Separations. ACS Appl. Mater. Interfaces 2011, 3, 4451−4458. (32) Cheng, H.; Sha, X.; Chen, L.; Cooper, A. C.; Foo, M.-L.; Lau, G. C.; Bailey, W. H., III; Pez, G. P. An Enhanced Hydrogen Adsorption Enthalpy for Fluoride Intercalated Graphite Compounds. J. Am. Chem. Soc. 2009, 131, 17732−17733. (33) Kashtiban, R. J.; Dyson, M. A.; Nair, R. R.; Zan, R.; Wong, S. L.; Ramasse, Q.; Geim, A. K.; Bangert, U.; Sloan, J. Atomically Resolved Imaging of Highly Ordered Alternating Fluorinated Graphene. Nat. Commun. 2014, 5, No. 4902. (34) Nicholson, R. S. Theory and Application of Cyclic Voltammetry for Measurement of Electrode Reaction Kinetics. Anal. Chem. 1965, 37, 1351−1355. (35) McCreery, R. L. Advanced Carbon Electrode Materials for Molecular Electrochemistry. Chem. Rev. 2008, 108, 2646−2687. (36) Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F. Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials. ACS Catal. 2014, 4, 3957− 3971. (37) Ambrosi, A.; Pumera, M. Precise Tuning of Surface Composition and Electron-Transfer Properties of Graphene Oxide Films through Electroreduction. Chem. - Eur. J. 2013, 19, 4748−4753. (38) Jankovský, O.; Mazánek, V.; Klímová, K.; Sedmidubský, D.; Kosina, J.; Pumera, M.; Sofer, Z. Simple Synthesis of Fluorinated Graphene: Thermal Exfoliation of Fluorographite. Chem. - Eur. J. 2016, 22, 17696−17703. (39) Teo, W. Z.; Sofer, Z.; Š embera, F.; Janoušek, Z.; Pumera, M. Cytotoxicity of Fluorographene. RSC Adv. 2015, 5, 107158−107165.
17706
DOI: 10.1021/acsomega.8b02582 ACS Omega 2018, 3, 17700−17706
