Electrochemical exfoliation-streamline method for synthesis of nitrogen doped graphene

INTRODUCTION

The goal of the paper is to explain the electrochemical exfoliation process for the synthesis of N-functionalized graphene as well as the characterization of the acquired material.

Beyond its unique set of physico-chemical properties, graphene can be considered as a robust, atomic scale scaffold from which other 2D materials can be derived through the replacement/attachment of foreign atoms and functional groups [1–3]. Among the  possible doping agents, nitrogen  (N) has drawn a considerable amount of attention because its atomic radius is comparable to that of carbon and contains five valence electrons available to form strong covalent bonds. The  conjugation between the nitrogen lone-pair electrons and the graphene π-system modifies graphene physical and chemical properties. The substitution of carbon atoms by nitrogen ones influences the atomic charge distribution on the  graphene scaffold and creates ‘active sites’ thus significantly increasing the electrochemical activity of nitrogen-doped graphene, known as N-graphene (NG) [4].

Graphene, as a  2D carbon nanomaterial in which sp2 hybridized carbon atoms are aligned in a  honeycomb lattice, is a  relatively new material with a wide range of applications due to its chemical stability, electric and thermal conductivity and large surface area [3–7]. Recent studies reveal that substitutional doping of graphene with heteroatoms N can tailor its electrical conductivity, wettability and electro-active surface area to facilitate charge-transfer and electrolyte–electrode interactions, and may give rise to its electrochemical performance [7, 8].

As mentioned before, the  functionalization of graphene improves upon its base qualities. In the  case of nitrogen doping resolve at least three types of C–N bonds (defect sites) can form during the functionalization process [7, 9]: quaternary N (or graphitic N), pyridinic N and pyrrolic N (Fig. 1) [10]. Usually, pyridinic N bonds with two C atoms at the edges or defects of graphene contribute one p electron to the π system. Pyrrolic N refers to N atoms that contribute two p electrons to the π system, although unnecessarily bond into the  five-membered ring, as in pyrrole. Quaternary N is the N atoms that substitute for C atoms in a hexagonal ring. In these nitrogen types, pyridinic N and quaternary N are sp2 hybridized and pyrrolic-N is sp3 hybridized [2, 3, 6, 10].

Nitrogen doped graphene can further be used in batteries, sensors, supercapacitors, fuel cells and more [2, 6, 7].

EXPERIMENTAL

Commercially available chemicals have been used. High-purity graphite rods (Alfa Aesar, 99.9995%), commercially available chemicals (analytical grade)-NaN₃, NaNO₂, HNO₃ and 2-propanol were used as supplied without additional purification. Distilled water was used for electrolyte preparation and washing.

Electrochemical exfoliation of graphite was carried out in a  800  ml beaker using two high-purity graphite rods (HOPG) and a  potentiostat (212, ISSP) situated as shown in Fig. 2. The synthesis procedure was carried out in a pulse mode of electrolysis-polarization of electrode by +10 and 0 V (both throughout 30 s) with multiple changing of polarization potential. The  synthesis process was carried out in 27 h maximum time. One molar aqueous solution of NaN₃ and NaNO₂ was used as electrolyte with different procedures and results. In the case of NaNO₂ about 75 ml of HNO₃ was added at the beginning of the process. The obtained dispersion of the  graphene material was stirred using ultrasound treatment in a UP200St ultrasonic processor (hielsher) for 1 min and then filtered through series of filters with different pore sizes (0.45, 0.2 and 0.1  µm). During filtration the material was rinsed thoroughly with distilled water and dried in an oven at 50°C.

For XPS measurements an ESCALAB Xi, Thermo Fisher spectroscope was used. For SEM images a  Helios 5  UX, Thermo Fisher Scientific microscope was used. For XRD measurements a MiniFlex 600, Rigaku spectroscope was used.

The dry material was then wrapped in filter paper and washed with a mixture of water and 2-propanol using a  Soxhlet extractor therefore getting rid of the unwanted leftover synthesis solution. After letting the process run for 15 h, the material was dried and, if needed, transferred to the appropriate solvent and dispersed using ultrasound treatment for 1 min.

RESULTS AND DISCUSSION

During the exfoliation process a change in the colour of the electrolyte from colourless to grey, then to black, could be observed, indicating that graphene sheets were created during the process [3]. During the electrolysis process azide decomposes into Na⁺ and N₃^– ions as well as sodium nitrite decomposes into Na⁺ and NO₂^– ions. The N₃^– ion changes further: N₃^–  →  3/2N₂+e^– and in water N₃^–+3H₂O+2e^– → N₂+NH₃+3OH^–. The nitrite ion reacts with water creating nitric acid and nitric oxide: NO₂^–+H₂O  →  HNO₃+NO and in further reactions N₂ and NH₃ are created. The  N₃^– and NO₂^– ions intercalate in the  graphite layers, and when they decompose, they simultaneously split the  graphite layers and functionalize the  graphene [3].

Figures 3, 4 and 5 show the X-ray photoelectron spectroscopy (XPS) measurements of graphene oxide GO synthesized by a  modified Hummer’s method [11] and NG from electrochemical exfoliation in NaN₃ and NaNO₂, respectively.

XPS is the standard quantitative spectroscopy technique to study the nitrogen doping effect in graphene. In the  XPS spectrum of N-graphene, at about 400 and 284  eV the  peaks appear that resemble N1s and C1s, respectively. The ratio of peak intensity between N1s and C1s is employed to determine the nitrogen content in N-graphene.

In Fig. 3 we can examine GO to determine that graphene has indeed been produced considering that graphitic carbon (Csp2 hybridized) can be viewed (284–285 eV) in all of the scans [7, 8, 12].

The N1s spectrum is used to determine the nitrogen configurations. Usually the  N1s spectrum can be deconvoluted to several individual peaks that are allocated to different C–N bonds: pyridinic-N (400  eV), pyrrolic-N (401–402  eV), quaternary-N (402–403 eV) and N-oxides of pyridinic N (408–410 eV) [4–6, 8, 9, 12, 13]. As a consequence of the  sample being charged, the  readings could change, as we see in Fig. 4, where all of the peaks have moved.

Figure 6 shows the X-ray spectroscopy (XRD) measurements of the  solution after the  electro chemical exfoliation in NaN₃. We found that the solution contained mainly NaHCO₃ [14] and we can assume that all NaN₃ has dissociated. We can assume that the C⁴⁺ ion presence tells us that the C–C bonds in graphene have been split and the lack of nitrogen in the solution tells us that there is a big likelihood that C–N bonds have been created.

Figure 7 shows the XRD measurements of gra-phene that was synthesized using NaNO₂ electrolyte solution. We can see a peak at around 26–28 degrees and that is consistent with the peak of nitrogen-doped graphene [5, 6, 12, 13].

Figure  8 is a  scanning electron microscope (SEM) image taken of nitrogen doped graphene that has been synthesized using NaN₃. We can see the single layer of graphene that has been formed (a) which confirms that the graphite has been exfoliated into graphene.

Furthermore, we see small particles of graphene at the size of 10 nm and even barely visible particles as small as 1 nm (b), in the image they can be seen as small white spots on a bigger graphene piece.

Figure 9 is a SEM image taken of nitrogen doped graphene that has been synthesized using NaNO₂. In the image (a) we see 50 nm graphene sheets that have not come apart fully. In the image (b) we see a  similar situation, only the  sheets are smaller at around 3 nm if we look at the smallest sheets.

In conclusion, the process and results of electrochemical preparation of multilayer graphene functionalized with nitrogen (N-MLG) via exfoliation of graphite electrodes in the presence of azide and nitrite anions in a pulse mode of electrolysis have been proven. Sodium azide and nitrite provide not only exfoliation of graphite into graphene but also simultaneously functionalize the graphene with nitrogen ions as a part of azide and nitrite ion decomposition. The X-ray diffractometer analysis confirmed that all of the nitrogen had left the solution after synthesis and C⁴⁺ ion was present which indicated that the nitrogen probably had intercalated into the graphene sheets. The  X-ray photoelectron spectroscope confirmed the nitrogen groups in the material and GO was used as a reference material. The scanning electron microscope images confirmed that few-layer graphene was acquired as well even though it could not have been filtered due to its small size. Further studies are needed to improve upon the synthesis method.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge financial sup port from the Latvian Council of Science, Project LZP FLPP No. LZP-2018/1-0194, and the Institute of Solid State Physics, University of Latvia that as the Center of Excellence has received funding from the  European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2026-2017-TeamingPhase2 under Grant Agreement No. 739508, Project CAMART².

References

Roberts Olins, Peteris Lesnicenoks, Janis Kleperis, Ainars Knoks, Ingars Lukosevics

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