Fabrication of pore-rich nitrogen-doped graphene aerogel

Abstract

Tuning the porosity of NGAs by tailoring the size of GONSs results in the pore-richest NGA with the best mechanical stability and electrocatalytic biosensing activity by using the smallest sonicated GONSs with DA as precursors, where DA favours high N content and 3D crosslinking capability.

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Graphene aerogels (GAs) are drawing more and more attention because of their high specific surface area,¹ rich pore network as an ion transportation pathway and potential applications in supercapacitors for energy storage and in oil–water separation.^2–4 Chemically doping nitrogen (N) atoms into GAs yields N-doped graphene aerogels (NGAs), showing improved performance of capacitance and electrocatalytic activity,^5,6 because the introduced N atoms intrinsically improve the electronic properties of graphene.^5,7 Adopting high N content of macromolecules to crosslink graphene in 3D direction, should improve the performance of NGAs. However, reported macromolecules including polypyrrole and polyaniline only contain –NH groups in backbone main chain.^3,8 Dopamine (DA) is a promising candidate. On one hand, it has high N content; on other hand, it can spontaneously polymerize into polydopamine (PDA), which is recently revealed be a universal adhesive.⁹ PDA contains –NH groups in both main chain and side chain, capable of separating and crosslinking the graphene oxide nanosheets (GONSs) against aggregation to form 3D GAs.

Physicochemical properties of GAs correlates with inner pore size,¹⁰ which has been tuned through varying the size of the sacrificial templates or changing reduction agents.^5,11 However, the properties of GONSs, the main precursors of the GAs or NGAs, including amphiphilicity, conductivity and transmittance closely depend on their size.^12,13 To obtain GONSs of certain sizes, many methods have been found including varying the size of precursor graphite,¹⁴ changing oxidation conditions,^15,16 and sonochemically exfoliating.^17,18 Among them, the sonochemical approach is remarkably efficient and low-cost. An important question arises whether the pore size and performance of the GAs or NGAs can be modulated by simply changing the size of GONSs, which has been forgotten in this fast growing area. What can be expected is that NGAs with smaller GONSs packing more densely with higher N-content should exhibit improved performance.

Herein, we emphasize an assembly strategy of NGAs, using different size and oxidization degree of sonicated GONSs with DA as precursors. GONSs packing more densely with smaller size and higher oxidization degree yield NGAs with higher N content, smaller pore size, richer pore structure, larger specific surface area, and better electrocatalytic detection of individual ascorbic acid (AA), DA, and uric acid (UA) in their ternary mixture at the same molar ratio as that in physiological fluids.

Suspensions of GONSs synthesized by an improved Hummers's method are subject to sonication for 0, 4, 8 h, respectively, to obtain different size and oxidation degree of GONSs, named as GONSs-0, GONSs-4, GONSs-8, respectively. Sonication effectively tailors the lateral size and exfoliates the stacked layers: both the lateral dimension and thickness of GONSs decrease with sonication time (Fig. 1a–c and Table S1†). The increased ratios of I_D/I_G (Fig. 1d), and the Csp³/Csp² (at/at) (Fig. S1†), as well as O/C (at/at) of GONSs (Table S1†) indicate the increase of oxidation degree of GONSs during sonication.¹⁹

Fig. 1 AFM height images and section analysis of (a) GONSs-0, (b) GONSs-4, and (c) GONSs-8. (d) Raman spectra of GONSs; (e) FTIR spectra of GONSs and NGAs. (f) XPS spectra of NGAs and GONSs-8. (g) Photographs of GAs and NGAs.

The NGAs with different pore sizes and N content are prepared by using different lateral sizes of sonicated GONSs with DA as precursors. Upon treatment of GONSs with DA in a hydrothermal process followed by annealing at 800 °C, a series of reactions occur, including the polymerization of DA to PDA, crosslinking of GONSs by PDA, and doping of N atoms from PDA into N-doped graphene oxide nanosheets (NGOSs). The appearance of N1s peaks (398.0 eV) confirms the N doping in graphene (Fig. 1f). The three different valence states of N atoms including pyridinic-N (∼398.2 eV), pyrrolic-N (∼399.1 eV), and quaternary-N (∼400.5 eV) in NGAs further confirm the doping of N atoms into the skeleton of graphene (Fig. S2†), which should improve the electrocatalytic activity of NGAs.⁵ The N (atom%) content of NGA-8 is higher than that of NGA-4 and NGA-0 (Fig. 1f and Table S2†), and also higher than those in reported 2D or 3D N doped graphene,^20,21 implying that NGA-8 should have the best electrocatalytic activity to be confirmed later.

Macroscopically, GAs and NGAs exhibit different size for a fixed amount of GONSs precursor. Compared with powder GAs from the pristine GONSs without DA (top photographs in Fig. 1g), the NGAs from GONSs with DA, exist as bulk aerogels with a stable shape (low row photographs in Fig. 1g). The compression strength increases from the 21.4 of NGA-0 to 28.7 of NGA-4 and to 44.6 kPa of NGA-8. The increased mechanical stability indicates that DA not only serves as the N source, but also as a strength intensifier due to the 3D crosslinking effect after polymerization into PDA.

Microscopically, NGAs appear as open 3D rich-pore network, which is different from macroporous graphene monoliths with closed pores from the soft template of hexane droplets.²² With the decrease of size and the increase of oxidization degree of the precursor GONSs, the NGOSs pack more densely in NGAs, resulting in the decreased macropore size of the NGAs (Fig. 2a–c and S3a–c†), more pores in the walls of NGAs (Fig. 2a–c and S3a–c†), more corrugated microstructures (Fig. 2d–f), as well as the increased packing layer number of NGOSs (Fig. 2g–i).

Fig. 2 SEM images of (a) NGA-0, (b) NGA-4, and (c) NGA-8. The TEM images of (d) NGA-0, (e) NGA-4, and (f) NGA-8. The HRTEM images of (g) NGA-0, (h) NGA-4, and (i) NGA-8. (j) N₂ adsorption–desorption isotherms of NGAs. (k) Pore size distribution of the NGAs calculated by DFT method.

The pristine graphene sheets tend to form slit-like nanopores due to corrugation, benefiting the large specific surfaces area for energy storage.² Here we show that the sizes of nanopores including the slit-like nanopores are well tuned by the precursor GONSs. With the size decrease of GONSs, both the average pore size and pore volume decrease (Table S2†), while the number of nanopores and micropores for a fixed amount of NGAs increases (Fig. 2j and k). The tunable porosity of the NGAs correspondingly tunes the BET specific surface area (Table S2†). The NGA-8 exhibits the largest BET specific surface area of 1020 m² g⁻¹ among the NGAs in this work, which is larger than the largest value (920 m² g⁻¹) of so far reported graphene-based porous materials.^20,23,24

Based on above discussion, the assembly process of NGAs can be briefly illustrated in Scheme 1. In the hydrothermal process, the polymerization of DA molecules into PDA, crosslinking of GONSs by PDA, and the doping of N atoms from PDA into NGOSs simultaneously occur. The π–π stacking interaction of the NGOSs, the hydrogen bonds and amide bonds between PDA and NGOSs result in the formation of 3D NGOSs/PDA hydrogel. Upon annealing at 800 °C, the PDA is totally pyrolysed and more N atoms are introduced into the graphene skeleton and NGAs consisting of only NGOSs are obtained. The higher the O content of the GONSs, the more oxygen-containing groups reacting with the PDA, and the higher N content doped in the skeleton of graphene. Smallest NGOSs stack the most densely, resulting in the largest layer number, and the smallest slit-pores and nanopores.

Scheme 1 Schematic assembly of NGAs with different porosity and N content from different size and oxidization degree of sonicated GONSs with DA as precursors.

The N atoms in the skeleton of graphene optimize local electronic structures and enhance the binding ability of electrolyte cations (K⁺, Na⁺) on graphene surface, facilitating the access of aqueous solution to the surface of the electrode, increasing the surface wettability (Fig. S4†).^7,25 High specific surface area exposes more active N atoms to solutions.^2,5 The electrocatalysis of AA or DA or UA on the GCE/NGAs is an adsorption-controlled process, as confirmed by a linear increase of current intensity for GCE/NGAs with increasing scan rates (Fig. S5–S7†).²⁶ So electrodes modified by the NGA-8 with the highest N content, the largest specific surface area, the highest electrical conductivity (Table S3†), and biggest electron transfer rate constant (k_s) calculated by the Laviron method (Table S3†),²⁷ should show the best electrocatalytic activity for simultaneous (or individual) determination of AA, DA, and UA, which cannot be determined by commercial GCE electrodes.

Compared with the completely overlap and totally indistinguishable oxidation peaks of AA, DA, and UA for the GCE and GCE/GAs, the three well separated oxidation peaks with larger peak current intensity (i_p) and peak potential (E_p) difference appear for the GCE/NGAs (Fig. 3a). The oxidation peak of AA and DA corresponds to the oxidation of hydroxyl groups to carbonyl groups.⁶ UA is firstly oxidized to quinonoid, and then converted to a tertiary alcohol or a carboxylic acid through reacting with water.²⁸ DA has higher π electron density than AA and UA due to the differences in their chemical structures (Fig. S8, ESI†), which results in stronger π–π interaction between DA and NGAs than that of AA or UA, corresponding to the larger i_p of DA than that of AA or UA (Fig. 3a).

Fig. 3 (a) DPV plots for 1.0 mM AA, 0.05 mM DA and 0.10 mM UA in 0.10 M PBS (pH = 7.4) for electrodes of GCE/NGAs, GCE/GAs, and GCE. DPV plots for different concentrations of DA containing 1.0 mM AA and 0.05 mM UA for electrodes of (b) GCE/NGA-0, (c) GCE/NGA-4 and (d) GCE/NGA-8.

Among the GCE/NGAs, GCE/NGA-8 has the best electrocatalytic activity including the highest i_p, smallest E_p and the largest E_p difference (Fig. 3a and Table S4†). This is of great importance for the practical simultaneous determination of AA, DA, and UA. The E_p (and i_p) for GCE/NGA-8 are almost half smaller (and 3 times) than the smallest E_p of AA and (the highest i_p of DA) for the reported graphene-based materials.^6,29

The concentration determination of AA, DA, and UA in their mixture is also performed by using the GCE/NGAs. The peak current of DA is linearly proportional to its concentration by keeping the concentrations of AA and UA constant, and no interference is observed for the determination of DA by the coexisting AA and UA (Fig. 3b–d). Similarly, by keeping the concentrations of AA (or UA) and DA constant, the peak current of UA (or AA) increases linearly with their concentrations (Fig. S9†). Concerning the detection limit, sensitivity and linear range, GCE/NGA-8 is also the best among GCE/NGAs for the individual measurement of AA, DA and UA (Table S5†). For GCE/NGA-8, the detection limit of AA is 40 times lower, the linear range of DA is 1.5 times larger, the linear range of UA is 4.0 times larger than the reported electrodes.^6,29–31

In summary, 3D NGAs with effectively tunable porosity can be assembled by simply using different size and oxidization degree of sonicated GONSs with DA as precursors. Increasing sonication time decreases the size and increases the oxidization degree of GONSs. DA providing N atoms, reducing, crosslinking and distributing the GONSs plays an important role in the formation of NGAs with higher mechanical strength than GAs without DA as precursor. The smallest GONSs packing the most densely enable the construction of corresponding NGAs with highest N content, smallest pore size, largest specific surface area, and best electrocatalytic activity. This work might benefit the preparation of exfoliated micro/nano-sheets by sonication, the modulation of 3D porous materials, and the application in highly sensitive bioassay.

Acknowledgements

We thank the National Nature Science Foundation of China (No. 21273135, 21573133 and 21403128), Shandong Provincial Natural Science Foundation, China (ZR2010BM039), Independent Innovation Foundation of Shandong University (2011JC026), and the Max Planck Society, Germany. We thank Prof. Möhwald for valuable remarks on the manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02049f

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