One-Step Electrochemical Preparation of Multilayer Graphene Functionalized with Nitrogen
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A new environmentally friendly one-step method for producing multilayer (preferably 7–9 layers) nitrogen-doped graphene (N-MLG) with a slight amount of oxygen-containing defects was developed. The approach is based on the electrochemical exfoliation of graphite electrode in the presence of azide ions under the conditions of electrolysis with pulse changing of the electrode polarization potential. It was found that usage of azide anions lead not only to the exfoliation of graphite but also to the simultaneous functionalization of graphene sheets by nitrogen atoms (as a result of electrochemical decomposition of azide anions with ammonia evolution). Composition, morphology, structure, and electrochemical properties of N-MLG were characterized by C,H,N analysis, transmission electron microscopy, atomic force microscopy, FTIR, UV–Vis, and Raman spectroscopy, as well as cyclic voltammetry. The perspective of using N-MLG as oxygen reduction reaction electrocatalyst and for the electrochemical analysis of biomarkers (dopamine, ascorbic acid, and uric acid) in their mixtures was shown.
KeywordsNitrogen-doped graphene Electrochemical exfoliation Sodium azide Electrocatalysis Electroanalysis
Atomic force microscopy
Electrochemically reduced graphene oxide
Fourier transform infrared spectroscopy
Electrochemically prepared multilayer graphene
Multilayer nitrogen-doped graphene
Oxygen reduction reaction
Transmission electron microscopy
Graphene, as a 2D carbon nanomaterial in which sp2-hybridized carbon atoms aligned in a honeycomb lattice, has attracted tremendous research interest due to its excellent electrical conductivity, high specific surface area, unique physical characteristics, mechanical properties, and chemical stability [1, 2, 3]. Functionalization of single-layer or multilayer graphenes by doping with different heteroatoms, in particular by nitrogen, allows controllable change electronic structure and, consequently, desirable properties of corresponding 2D materials . It opens up new opportunities for creating of multifunctional nanostructured carbon materials, dispersions, and hybrid composites, used in catalysis, power engineering, biomedicine, “smart” materials and systems, etc.
Nitrogen-doped graphene (N-graphene) can be prepared via direct incorporation of nitrogen atoms into graphene by means of, for example, chemical vapor deposition, arc discharge, and solvothermal processes [5, 6, 7] or by N-doping of initially prepared graphene oxide (graphene) under thermal, plasma, electrochemical, etc. post-treatment [8, 9, 10, 11]. In the first case, the main disadvantages are the harsh reaction conditions, sufficiently long duration, and high cost of processes due to using of specific equipment and necessity of strict implementation of manufacturing operations. Major drawbacks of the second approach are the multistaging of process and the using of environmentally hazardous reagents.
The electrochemical exfoliation of graphite is a promising approach to produce graphene and graphene-related materials due to its easy, fast, and environmentally friendly nature . Recently, one-step production of multilayer N-graphene by electrochemical exfoliation of graphite electrode in aqueous electrolytes based on protic ionic liquid (ethylammonium nitrate) , ammonium nitrate , or (NH4)2SO4 and NH4OH  were reported. However, despite the advantages of the proposed approaches, their wide use is limited by the high cost of ionic liquid , high content of unwanted oxygen-containing defects in N-graphene [12, 13, 14], environmentally adverse concentrated solution of ammonia, using a sufficiently high potential, and a prolonged ultrasonic treatment .
Previously, we have shown the possibility of the formation of multilayer graphene (MLG) with slight amounts of oxygen-containing defects by means of exfoliation of graphite electrode in presence of benzoate anions in a pulse mode of electrolysis . It is supposed that usage of azide anions instead of carboxylate anions can lead not only to the exfoliation of graphite but also to the simultaneous functionalization of graphene sheets by nitrogen atoms (as a result electrochemical decomposition of azide anions).
Considering the above, the purpose of this study was to establish the possibility of electrochemical one-step production of N-graphene with slight amounts of oxygen-containing defects via exfoliation of graphite in an aqueous solution of sodium azide in a pulse mode of electrolysis without using concentrated ammonia solution, expensive ionic liquids, and high potentials, as well as clarification of its electrocatalytic activity in the oxygen reduction and oxidation of such biomarkers as ascorbic acid (AA), dopamine (DA), and uric acid (UA).
Chemicals and Materials
High-purity graphite rods (Alfa Aesar, 99.9995%), gasses (Ar and O2), and commercially available chemicals (analytical grade)—NaN3, KCl, H2SO4, ascorbic acid, dopamine, and uric acid—were used as supplied without additional purification. The distilled water was used for electrolyte preparation. The graphene oxide (GO), used in the study for comparison, was obtained via liquid phase exfoliation of graphite oxide, synthesized by the modified Hummers method .
Electrochemical studies were carried out via computer complex based on potentiostat PI-50-1.1 using a three-electrode undivided cell (working electrode—glassy carbon (GC) disk with visible surface area of 0.03 cm2; the auxiliary electrode—platinum mesh; reference electrode—Ag/AgCl, 3 M KCl). In order to modify electrode, 2 μL of aqueous or alcoholic dispersion (1 mg/mL) of the corresponding graphene material was dropped onto its surface, followed by drying on air. TEM images were recorded using a transmission electron microscope TEM125K (Selmi) with an accelerating voltage 100 kV (samples were deposited onto copper grids coated with amorphous carbon film). Atomic force microscopy (AFM) of thin film graphene samples on the surface of silicon wafers coated with silicon nitride (Agar Scientific) was performed on a Nanoscope IIIa Dimension 3000TM (Digital) instrument. FTIR spectra were taken on Fourier transform infrared spectroscope SPECTRUM ONE (PerkinElmer); samples were prepared as tablets with KBr. UV–Vis spectra of dispersions were registered via UV–visible spectrometer 4802 (Unico). Raman spectra were obtained with a triple spectrometer (Horiba Jobin-Yvon T64000, Ar–Kr laser, λ = 514 nm); samples were deposited onto silicon templates. C,H,N-elemental analysis was performed on Carlo Erba 1106 elemental analyzer (Carlo Erba, Italy) based on modification of the classical Pregl and Dumas method (combustion temperature of 1030 °C, atmosphere of oxygen) using 0.5–1.0 mg of sample per analysis. The oxygen content in the samples was evaluated by difference between the total weight of the samples and content C,H,N in them (on the basis of C,H,N analysis data).
Results and Discussion
During electrolysis, it was observed the gradual change of electrolyte color from colorless to gray and then to dark gray (Fig. 1b), which indicates the transition of graphene sheets in the electrolyte volume as a result of exfoliation of the graphite electrode. It is believed that during positive electrode polarization, the intercalation of azide anions (N3 −) into graphite interlayer space followed by its partial anodic decomposition, N3 − → 3/2 N2 + e− , take place. In case of application to electrode of potential 0 V, the deintercalation of N3 − occurs and also its partial decomposition: N3 − + 3H2O + 2e− → N2 + NH3 + 3OH− . Multiple repetition of anion intercalation/deintercalation cycles into graphite interlayer space as well as N2, NH3, and O2 evolution during electrolysis provide separation of graphene layers, forming multilayer packages of graphene, doped with nitrogen, that are passing to electrolyte volume. At the same time, ammonia, evolved as a result of cathodic decomposition of N3 −, acts as a nitrogen source for the in situ graphene doping, while using of low potentials and nitrogen evolution as a result of electrochemical process promotes low number of oxygen-containing defects in N-graphene. Furthermore, over time pH of the electrolyte was changed from 7 to 11–12 (Fig. 1c), confirming the formation of ammonia and hydroxyl anions—as a result of the partial electrochemical decomposition of azide ions, that is evidence in favor assumption, made above, about the mechanism of the process under the used conditions.
Partially oxidized state of obtained material and functionalization of it by nitrogen were also confirmed by C,H,N-analysis data. Thus, calculated nitrogen content in N-MLG was about 0.6% and atomic ratio C/O ~17. Such C/O ratio indicates that oxygen-containing groups although are present in obtained N-MLG, but their content is much less than for example in chemically or thermally reduced GO, where C/O ~8–11 . It is important to note that increase of electrolyte concentration from 0.1 to 2 M leads to symbate increase of nitrogen content in resulting material from 0.2 to 0.9%, which opens up the perspectives of controlling the nitrogen content in multilayer graphene, obtained by proposed method.
The presence of band with maximum absorption at 267 nm in UV–Vis spectrum of N-MLG dispersion in ethanol (Fig. 4b), which corresponds to the so-called van Hove singularity in the graphene density of states , evidenced in support a slight oxidation of the obtained N-functionalized graphene. At the same time, UV–Vis spectrum of highly oxidized GO dispersion (Fig. 4b) differs from investigated UV–Vis spectrum of N-MLG: maximum of adsorption is observed at 242 nm as well as a shoulder at about 300 nm associated with the nπ*-junction with the participation of unshared electron pairs of the oxygen atoms in oxygen-containing groups .
In conclusion, the possibility of one-step electrochemical preparation of multilayer graphene functionalized with nitrogen (N-MLG) via exfoliation of graphite electrode in presence of azide anions in a pulse mode of electrolysis was presented. Sodium azide provides not only exfoliation of graphite via multiple repetition of anion intercalation/deintercalation cycles into graphite interlayer space but also simultaneous functionalization of graphene sheets by nitrogen atoms as a result of partial electrochemical decomposition of azide anions with ammonia evolution. Particles of N-MLG preferably consist of 7–9 individual graphene layers with a low amount of oxygen-containing defects (C/O ratio ~17), which was proved by means of C,H,N-analysis, TEM, AFM, FTIR, UV–Vis, and Raman spectroscopy. It was shown that increase of electrolyte concentration during electrochemical synthesis (from 0.1 to 2.0 M) allows change nitrogen content from 0.2 to 0.9% in resulting material.
It was found that N-MLG is a promising electrode material. By means of cyclic voltammetry, it was shown higher electrocatalytic activity of N-MLG in the oxygen reduction reaction, which is realized in fuel cells, compared to electrochemically prepared multilayer graphene or electrochemically reduced graphene oxide, that do not contain nitrogen atoms in their structure. Also, N-MLG was shown to be electrochemically active toward oxidation of such biomarkers as dopamine (DA), ascorbic (AA), and uric (UA) acids. Significant difference of oxidation potentials of DA, AA, and UA (when all three substances were present in the electrolyte simultaneously) suggests the possibility of using N-MLG in electroanalysis of mentioned above biomarkers in biological liquids.
The work was partly financially supported by the Target Complex Programs of Scientific Research of National Academy of Sciences of Ukraine “Fundamental problems of production of new chemical substances and materials” (project no. 14) and “Fundamental aspects of renewable-hydrogen energy and fuel cell technology” (project no. 24). The authors are grateful to Dr. A. Korchovyi and Y. Stubrov (V.E. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine) for the technical assistance in the AFM and Raman spectra measurements.
OU performed the synthesis and characterization of N-MLG, interpreted the experimental data, and drafted the manuscript. YK participated in the design of the study, interpreted the experimental data, and drafted the manuscript. VK conceived of the study and helped to draft the manuscript. VP participated in the design of the study and its coordination and helped to draft the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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